The White House expressed confidence in the chief of the US Secret Service, but the elite bodyguard unit’s Colombian sex scandal deepened, with claims that 20 women were involved.

We don’t know who these women are. They could be spies, they could be associated with hostile forces, they could have disabled the agents weapons or planted listening devices- senator

Secret Service director Mark Sullivan is coming under increasing political pressure over allegations that agents sent to the Caribbean resort of Cartagena ahead of President Barack Obama’s weekend visit consorted with prostitutes.

“The President has confidence in the director of the Secret Service,” White House spokesman Jay Carney said.

“Secret Service director Sullivan acted quickly in response to the incident and is overseeing an investigation as we speak into the matter,” Mr Carney said.

But Mr Obama’s spokesman did not directly respond to a question over whether Mr Sullivan should step down if the probe finds Secret Service agents acted improperly ahead of the Summit of the Americas.

“I am not going to, as the President did not speculate about conclusions the investigation might reach since it is ongoing,” said Mr Carney.

Mr Obama has said he would be “angry” if the allegations are proven true and has stressed US officials of all levels had a duty to behave properly when abroad representing their country.

US Republican Senator Susan Collins, meanwhile, offered new details after a briefing from Mr Sullivan on the alleged incident, which became public soon after Mr Obama arrived in Colombia for the Summit of the Americas.

“He told me that there are 11 Secret Service agents and officers involved,” she said in a statement.

“Some 20 women foreign nationals were brought to the hotel in (Colombia) but allegedly marines were involved with the rest.”

The Pentagon said that five military personnel were also under investigation.

Ms Collins later told reporters that some of the agents involved “were uniformed personnel who were assigned to building security. Others were these specialised agents who do security details”.

She warned that the security implications of the incident were “potentially extremely serious” amid fears the agents exposed themselves to blackmail.

“We don’t know who these women are. They could be spies, they could be associated with hostile forces, they could have disabled the agents weapons or planted listening devices,” she said.

Ms Collins, the ranking Republican on the Homeland Security and Governmental Affairs Committee said Sullivan told her that the 11 recalled Secret Service agents had been interviewed about the incident.

A government source said on Sunday that the agents had seen their top secret security clearances suspended pending the result of the investigation.

The incident has revived questions about the leadership of the Secret Service, which has an official mandate to protect the President and his family and also works to combat currency counterfeiting and fraud.

Mr Sullivan earlier found himself in hot waters in 2009 after socialite and reality star Michaele Salahi and her husband Tareq breezed through White House security at a state dinner for Indian Prime Minister Manmohan Singh.

He admitted mistakes were made over the security breach as he was grilled by lawmakers on Capitol Hill.

Senator Joe Lieberman, chair of the Homeland Security committee, said he would be prepared to hold a hearing if it would be productive.

“But I don’t want to just go ahead and do a hearing unless I think we can get something valuable out of it,” he told reporters.

Hundreds of Secret Service agents typically travel with, or before Mr Obama when he goes abroad, in a huge security retinue.

The agency insists on high standards of training and discipline for both its iconic plain clothes, presidential protective and uniformed squads.

But the latest stories have revived questions about the culture in the service and speculation about an alleged “wheels up, rings off” attitude of some travelling agents which officers say tarnishes the majority of the service.

The Secret Service has been responsive to such questions in the past, seeking to boost recruitment among women and ethnic minorities.