Colombia's government says it is close to sealing an agreement with Washington to make Colombia a hub for US anti-drug operations in South America.
The deal would give the US access to air bases in Colombia to gather intelligence and support operations against drugs production and terrorism.
Colombian officials have rejected accusations that the deal would infringe the country's sovereignty.
Colombia is the biggest recipient of US military aid in the Americas.
The US was forced to look for a new centre for regional operations after Ecuador refused to renew the lease on its military base of Manta.
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.
Final details of the Colombian plan are still being worked out but the US may use at least three air bases, including Malambo in the north, and Palanquero and Apiay in the central region.
A maximum of 800 US military personnel and 600 civilian defence contractors would be based in Colombia.
They would operate the US aircraft that maintain 24-hour monitoring of the region, intercepting communications and coordinating with spy satellites to protect US interests.
The US mission is supposed to be primarily against drug-trafficking, but also has a counter-terrorism element as Colombia's rebels groups are on the US list of terrorist organisations.
Colombia may well be a close ally of the US, but this latest deal is causing controversy, says the BBC's Jeremy McDermott in Medellin.
Rafael Pardo, the presidential candidate for the Liberal Party, said he was concerned about how relations with neighbouring countries would be affected because the US aircraft operating from these bases would probably be collecting intelligence on these nations.
But the Colombia Foreign Minister Jaime Bermudez said: "The aim is to defeat drug-trafficking and terrorism."
He said the agreement would respect Colombia's neutrality and sovereignty, and would not affect ties with neighbouring countries.