Ecuador's parliament has approved a law banning foreign military bases, a move which could prevent the US from using a key anti-drug smuggling base in the country.
Rafael Correa, Ecuador's president, whose party controls the assembly, had previously said he would not renew the agreement allowing US forces to operate from the city of Manta.
At present the US lease on the Pacific coast base expires in 2009.
US officials say that air surveillance missions from the base have led to more than half of all drug seizures in the region, where most of the world's cocaine is produced.
'Compromising sovereignty'
Ecuador's 130-member assembly, controlled by Correa's Alianza Pais party, passed the move in the first package of rewrites to the country's new constitution, which Ecuadoreans will vote on later this year.
"Ecuador is a land of peace [and] foreign military bases or foreign installations with military purposes will not be allowed," the amendment said.
If ratified in the referendum, the amendment would probably end any chance of the US being able to negotiate further use of the base.
Correa's party says that US operations at the base compromise the country's sovereignty.
"Of course we're going to follow the line of the new constitution if it is approved," Linda Jewell, the US ambassador to Ecuador, said.
"We are going to talk with the government to see how we can continue working in the fight against drug trafficking."
Ecuador produces very little cocaine, but is often used as a transit point for drugs sent from neighbouring Colombia and Peru - the world's top two cocaine producers.