This sounds very much like Interceptor Force (also known as "Interceptors") from 1999, and its low-budget sequel Interceptor Force 2.

The IMDB summary for the first film agrees with the main points in the question. An elite team are sent to Mexico to investigate reports of an alien landing:

In the near future, the government has created an elite team of soldiers specially trained in the event of a hostile alien encounter. A multi-national force of the world's best, they are known as the Interceptor Force. A covert early warning satellite network tracks an object crash landing in a town in the Pacific Northwest. Within minutes of the impact, all communication with the town is mysteriously severed. The news sources are given a false story of contamination and the area is sealed off. AWAC's and fighters patrol the perimeter. As the silence from the town continues, the Interceptor team lead by Lieutenant Sean Lambert (Gruner) is sent in to investigate. An MC 130 Talon is scrambled for Lambert and his team to make the high altitude air drop. Once inside, the team encounters an alien race capable of assuming human likeness, hiding their reptilian form. Simultaneously the government has given the team less than 24 hours to terminate this alien force before launching an air strike with tactical nukes to completely sterilize the area. Lambert and his team now must battle this alien military force as the clock ticks and a squadron carrying certain death approaches.

In the sequel from 2002, the alien's mate arrives on Earth seeking revenge. Again, from the imdb summary:

In the not too distant future a new team of specially trained government soldiers called an Interception Team contend with the latest hostile alien encounter. When an alien scout ship crashes in eastern Russia, Sean Lambert and his elite force are dispatched to investigate. They come face-to-face with a seemingly invincible seven-foot tall, shape shifting foe hell-bent on blowing up a Russian nuclear power plant in an effort to destroy mankind through a nuclear winter. The stakes become personal when the alien discovers it was Lambert who killed its mate four years ago (Interceptor Force). A major battle ensues as the Interceptor Team look to save the Earth from alien annihilation.

The alien is impervious to all the weapons used on it, but is eventually defeated when it dissolves in a pool of toxic liquid.

This sounds very much like Interceptor Force (originally released as just "Interceptors") from 1999, and its low-budget sequel Interceptor Force 2.

The IMDB summary for the first film agrees with the main points in the question. An elite team are sent to Mexico to investigate reports of an alien landing:

In the near future, the government has created an elite team of soldiers specially trained in the event of a hostile alien encounter. A multi-national force of the world's best, they are known as the Interceptor Force. A covert early warning satellite network tracks an object crash landing in a town in the Pacific Northwest. Within minutes of the impact, all communication with the town is mysteriously severed. The news sources are given a false story of contamination and the area is sealed off. AWAC's and fighters patrol the perimeter. As the silence from the town continues, the Interceptor team lead by Lieutenant Sean Lambert (Gruner) is sent in to investigate. An MC 130 Talon is scrambled for Lambert and his team to make the high altitude air drop. Once inside, the team encounters an alien race capable of assuming human likeness, hiding their reptilian form. Simultaneously the government has given the team less than 24 hours to terminate this alien force before launching an air strike with tactical nukes to completely sterilize the area. Lambert and his team now must battle this alien military force as the clock ticks and a squadron carrying certain death approaches.

In the sequel from 2002, the alien's mate arrives on Earth seeking revenge. Again, from the imdb summary:

In the not too distant future a new team of specially trained government soldiers called an Interception Team contend with the latest hostile alien encounter. When an alien scout ship crashes in eastern Russia, Sean Lambert and his elite force are dispatched to investigate. They come face-to-face with a seemingly invincible seven-foot tall, shape shifting foe hell-bent on blowing up a Russian nuclear power plant in an effort to destroy mankind through a nuclear winter. The stakes become personal when the alien discovers it was Lambert who killed its mate four years ago (Interceptor Force). A major battle ensues as the Interceptor Team look to save the Earth from alien annihilation.

The alien is impervious to all the weapons used on it, but is eventually defeated when it dissolves in a pool of toxic liquid.

This sounds very much like Interceptor Force (also known as "Interceptors") from 1999, and it's low-budget sequel Interceptor Force 2.

The IMDB summary for the first film agrees with the main points in the question. An elite team are sent to Mexico to investigate reports of an alien landing:

In the near future, the government has created an elite team of soldiers specially trained in the event of a hostile alien encounter. A multi-national force of the world's best, they are known as the Interceptor Force. A covert early warning satellite network tracks an object crash landing in a town in the Pacific Northwest. Within minutes of the impact, all communication with the town is mysteriously severed. The news sources are given a false story of contamination and the area is sealed off. AWAC's and fighters patrol the perimeter. As the silence from the town continues, the Interceptor team lead by Lieutenant Sean Lambert (Gruner) is sent in to investigate. An MC 130 Talon is scrambled for Lambert and his team to make the high altitude air drop. Once inside, the team encounters an alien race capable of assuming human likeness, hiding their reptilian form. Simultaneously the government has given the team less than 24 hours to terminate this alien force before launching an air strike with tactical nukes to completely sterilize the area. Lambert and his team now must battle this alien military force as the clock ticks and a squadron carrying certain death approaches.

In the sequel from 2002, the alien's mate arrives on Earth seeking revenge. Again, from the imdb summary:

In the not too distant future a new team of specially trained government soldiers called an Interception Team contend with the latest hostile alien encounter. When an alien scout ship crashes in eastern Russia, Sean Lambert and his elite force are dispatched to investigate. They come face-to-face with a seemingly invincible seven-foot tall, shape shifting foe hell-bent on blowing up a Russian nuclear power plant in an effort to destroy mankind through a nuclear winter. The stakes become personal when the alien discovers it was Lambert who killed its mate four years ago (Interceptor Force). A major battle ensues as the Interceptor Team look to save the Earth from alien annihilation.

The alien is impervious to all the weapons used on it, but is eventually defeated when it dissolves in a pool of toxic liquid.

This sounds very much like Interceptor Force (also known as "Interceptors") from 1999, and its low-budget sequel Interceptor Force 2.

The IMDB summary for the first film agrees with the main points in the question. An elite team are sent to Mexico to investigate reports of an alien landing:

In the near future, the government has created an elite team of soldiers specially trained in the event of a hostile alien encounter. A multi-national force of the world's best, they are known as the Interceptor Force. A covert early warning satellite network tracks an object crash landing in a town in the Pacific Northwest. Within minutes of the impact, all communication with the town is mysteriously severed. The news sources are given a false story of contamination and the area is sealed off. AWAC's and fighters patrol the perimeter. As the silence from the town continues, the Interceptor team lead by Lieutenant Sean Lambert (Gruner) is sent in to investigate. An MC 130 Talon is scrambled for Lambert and his team to make the high altitude air drop. Once inside, the team encounters an alien race capable of assuming human likeness, hiding their reptilian form. Simultaneously the government has given the team less than 24 hours to terminate this alien force before launching an air strike with tactical nukes to completely sterilize the area. Lambert and his team now must battle this alien military force as the clock ticks and a squadron carrying certain death approaches.

In the sequel from 2002, the alien's mate arrives on Earth seeking revenge. Again, from the imdb summary:

In the not too distant future a new team of specially trained government soldiers called an Interception Team contend with the latest hostile alien encounter. When an alien scout ship crashes in eastern Russia, Sean Lambert and his elite force are dispatched to investigate. They come face-to-face with a seemingly invincible seven-foot tall, shape shifting foe hell-bent on blowing up a Russian nuclear power plant in an effort to destroy mankind through a nuclear winter. The stakes become personal when the alien discovers it was Lambert who killed its mate four years ago (Interceptor Force). A major battle ensues as the Interceptor Team look to save the Earth from alien annihilation.

The alien is impervious to all the weapons used on it, but is eventually defeated when it dissolves in a pool of toxic liquid.