Three US Secret Service agents have quit over a widening sex scandal in Colombia, as President Barack Obama got his first briefing on the notorious incident from the agency's director.

"In addition to the previously announced personnel actions, three additional employees have chosen to resign," Assistant Director Paul Morrissey said on Friday in a statement.

The statement said that one employee had been cleared of misconduct, but would face administrative action.

"The Secret Service continues to conduct a full, thorough and fair investigation, utilizing all investigative techniques available to our agency," the statement said.

The new resignations bring to six the number of agents who have lost their jobs over claims that they brought prostitutes back to their hotel in the Caribbean resort of Cartagena, host of last weekend's Summit of the Americas.

Obama trip shadowed by scandal

A twelfth member of the elite protection agency has now been implicated in the incident, a deep diplomatic embarrassment for the United States that overshadowed Obama's visit to Colombia.

Obama has said he will be "angry" if details of the scandal aired in the press are true.

He got his first briefing on the incident from Secret Service Director Mark Sullivan on Friday afternoon in the Oval Office, a senior US official said.

The US military earlier said it was now investigating 11 service members linked to events in Colombia.

Six suspects were from the US Army's 7th Special Forces Group, two from the Marine Corps, two from the Navy and one from the Air Force, said a spokesman for US Southern Command, which oversees forces in Central and South America.

An Air Force colonel in charge of the investigation flew to Colombia on Monday to gather facts in the case and was due return to the United States probably over the weekend. He then will interview the suspects.