Colombia has arrested two US soldiers on suspicion of trafficking weapons to right-wing paramilitary groups.
The US says its military is helping Colombia combat drug smuggling
Paramilitaries are accused of drug trafficking and mass killings during Colombia's 40-year civil conflict.
Officials said the US soldiers were arrested on Tuesday along with several Colombians in an operation south-west of the capital, Bogota.
Hundreds of American soldiers are in Colombia to help the Bogota government in their operation against drugs.
In March five US troops were arrested for allegedly trying to smuggle cocaine to America in a military aircraft.
They are now back on US soil, and it is thought that several are still under investigation.
The US embassy in Bogota said it was trying to establish exactly what had happened in the latest incident.
One unconfirmed report said more than 30,000 missiles had been seized in Tuesday's raid.
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.
Colombia is the third biggest recipient of US military aid, after Israel and Egypt.