The US Secret Service said it had put 11 agents on administrative leave to investigate their behaviour ahead of President Barack Obama's trip to a Colombia summit, and apologised for the distraction the incident had caused.

In a statement, assistant director Paul Morrissey said the Secret Service replaced the agents after allegations were made on Thursday, in line with the Secret Service's "zero tolerance" policy on personal misconduct.

"This is standard procedure and allows us the opportunity to conduct a full, thorough and fair investigation into the allegations," he said, adding: "These actions have had no impact on the Secret Service's ability to execute a comprehensive security plan for the president's visit to Cartagena."     

The Secret Service gave no details of what may have occurred in Cartagena, a coastal city hosting the 33-nation Summit of the Americas.

A local Colombian police source and The Washington Post said the agents brought prostitutes back to their hotel. "This incident is not reflective of the behaviour of our personnel as they travel every day throughout the country and the world performing their duties in a dedicated, professional manner," Morrissey said. "We regret any distraction from the Summit of the Americas this situation has caused."     

He said the personnel involved were special agents and uniformed division officers, none of whom were assigned to the presidential protective service. All 11 were interviewed at Secret Service headquarters in Washington on Saturday.

Obama arrived in Cartagena for the conference on Friday and will stay until Sunday.  - Reuters