Image caption David Chaney (right), one of the agents implicated, was on Sarah Palin's Secret Service detail in 2008

The US Secret Service has said three more employees are resigning over the prostitution scandal that overshadowed a summit in Colombia last weekend.

Supervisors David Chaney and Greg Stokes are among three agents already leaving the elite agency in the wake of the affair.

US President Barack Obama was briefed on Friday by Secret Service Director Mark Sullivan about the scandal.

It broke as the president arrived for last weekend's Summit of the Americas.

On Friday, a 12th Secret Service employee was placed on administrative leave.

Another member of staff at the agency was cleared by investigators of "serious misconduct", but will face administrative action.

The scandal broke when a dispute between an escort and an agent spilled into the hallway of a beachfront hotel.

Check this out, bodyguard: you're fired! Sarah Palin

Up to 20 women were involved in the antics in the city of Cartagena.

A 24-year-old Colombian single mother told the New York Times on Wednesday that an agent had agreed to pay her $800 (£500) for sex, but offered her only $30 the next morning.

Mr Chaney, 48, who was in the international programmes division, will be allowed to retire, but Mr Stokes, an assistant special agent in charge of the K9 division, has been told he will be fired, US media report. A third unnamed employee resigned over the allegations.

Lawrence Berger, a lawyer for Mr Chaney and Mr Stokes, told AP on Friday: "Nothing that has been reported in the press in any way negatively or adversely impacted the mission of that agency or the safety of the president of the United States."

Lawmakers on a congressional panel investigating the scandal had earlier warned more agents would lose their jobs.

Representative Elijah Cummings, the top Democrat on the House Oversight and Government Reform Committee, told Reuters news agency: "It would not surprise me if there were within the next few days additional resignations or firings."

Image caption Up to 20 women were involved in the antics in the city of Cartagena

A photo on Mr Chaney's Facebook page showed him near Sarah Palin while on her security detail during the former Alaska governor's 2008 vice-presidential run.

A comment apparently posted by him on the page said: "I was really checking her out, if you know what i mean?"

Mrs Palin took to Fox News on Thursday to respond.

"This agent who was kind of ridiculous there in posting pictures and comments about checking someone out," she said. "Check this out, bodyguard: you're fired! And I hope his wife sends him to the doghouse."

Eleven military members who were supporting the Secret Service in Colombia are also under investigation: six from the Army, one from the Air Force and two each from the Marines and Navy.