President Correa would rather "cut off his arm" than see the US stay
Ecuadorian lawmakers have approved a constitutional change that would outlaw foreign military bases on its soil.
The approval throws into doubt the future of a key US base in the South American country.
The US has its only South American base in the town of Manta but its 10-year lease is up for renewal next year.
The lawmakers' decision, if given final approval in a public vote, could signal the end of joint Ecuadorean and US efforts to fight drug cartels.
"Ecuador is a land of peace; foreign military bases or foreign installations with military purpose will not be allowed," read the amendment approved by the assembly, which is controlled by President Rafael Correa's Alianza Pais party.
The air base at Manta has great strategic value for the US military.
American officials say surveillance flights from Manta have led to more than half the illegal drug seizures in the region.
The coastal town also doubles up as a strategic look-out post for US forces monitoring warships heading north from the Middle East and Asia.
But Ecuador's left-wing president Rafael Correa, a political ally of Venezuelan President Hugo Chavez, has said he would rather "cut off his arm" than allow the Americans to stay on at Manta.
The move to ban foreign bases in Ecuador was first proposed by the country's constituent assembly last month.
It is one of several constitutional changes to be put to a national vote later in the year.
The dispute over the base comes during a period of strained relations between Ecuador and its neighbour Colombia - the US's closest ally in the region.
Tension almost boiled over last month following a Colombian military raid inside Ecuador in which a top commander of the Colombian rebel group, Farc, was killed.