Colombia has handed over to the US two American soldiers suspected of trafficking weapons to paramilitaries.
The US says it finds the allegations very "troubling"
The United States said the allegations were "extremely troubling" and would be investigated fully.
Colombian authorities said they found 31,000 rounds of ammunition when they arrested the two men at an apartment near Bogota on Tuesday.
Hundreds of American soldiers are in Colombia to help in the fight against paramilitary groups and the drug trade.
Right-wing paramilitaries are accused of drug trafficking and mass killings during Colombia's 40-year civil conflict.
The US would not identify the two soldiers.
"There is absolutely no US policy and US support or US inclination or US military operations involved in arming paramilitaries," state department spokesman Richard Boucher said.
"Allegations that the US military personnel involved were trafficking in ammunition are, indeed, extremely troubling to us."
"As with all criminal allegations against US military personnel, we're committed to a full investigation and we'll be working with the Colombian authorities in that regard."
The men are expected to be flown to the US in the next few days.
Colombian Vice President Francisco Santos said he hoped the men "will be judged and that the punishments will be similar to what they would receive here".
A senior Colombian official, Inspector General Edgardo Jose Maya, tried to have their deportation delayed, and asked for further examination of a treaty granting them immunity in Colombia.
Since 2000, the US has been funding an aid package known as Plan Colombia, under which Colombian forces receive training, equipment and intelligence to root out drug traffickers and eliminate coca crops.