The Secret Service said on Thursday it is investigating explosive new allegations of misconduct in El Salvador, broadening a sex scandal that a US lawmaker said may lead to changes in how agents do their work abroad.

A CBS broadcast affiliate in Seattle aired new accusations emerging from the Central American nation, where members of a presidential advance team one year ago reportedly were seen by a government subcontractor paying for sex at a strip club and taking escorts back to their hotel rooms.

They are the latest claims to hit the presidential protection force still reeling from a Colombia sex scandal that saw eight agents dismissed, and follow revelations that three US Marines and a State Department employee were punished for involvement with a prostitute in Brazil.

"Right now they have begun to question supervisors that were on the ground, personnel attached to the embassy," Representative Sheila Jackson Lee told CNN after a briefing with under-fire Secret Service director Mark Sullivan.

"They have not completed an investigation, but they really need more facts and they really are seeking for anyone who would come forward."

The Secret Service sent a memo to members of Congress Thursday saying it was looking into the allegations, and any credible information 'will be followed up on in an appropriate manner,' a congressional source told AFP.

KIRO 7 Eyewitness News quoted a subcontractor who worked 'extensively' with the Secret Service advance team and some military specialists, and spent time with them at a popular San Salvador strip club days before President Barack Obama's trip to the country in March 2011.

"Our witness tells us he repeatedly saw the Secret Service agents exchange money for sexual favors within the club, and on at least two occasions, those agents took escorts back to their hotel rooms," Chris Halsne, the investigative reporter who broke the story, said on CBS.

He also said the club owner verified that Secret Service agents had been there that week. The White House was mum on the allegations.

"I simply don't have anything for you on that from here," Obama spokesman Jay Carney told reporters.

Asked whether the president previously knew of the claims, he said: "I don't know that any of us were aware of it until we read newspaper reports."

While the Pentagon is investigating 12 military personnel allegedly involved in the Colombia events, Defense Secretary Leon Panetta said 'there's no investigation ongoing' regarding the El Salvador accusations.

Two days earlier, he said three US Marines and an embassy staffer in Brazil 'were severely punished' and sent home over an incident involving a prostitute, another potential black eye in the broadening scandal.

The events in Colombia, where agents allegedly brought sex workers back to their hotel rooms in Cartagena, have brought deep embarrassment to the agency in an election year.

At a Senate hearing on Wednesday, Homeland Security chief Janet Napolitano said she and Obama had full confidence in Sullivan, but pledged to 'leave no stone unturned' in her investigation.

Napolitano said she found no evidence of an agency-wide problem, insisting there have been no complaints of similar misbehavior at any time during the past two and a half years, a period during which the agency provided protection on more than 900 foreign trips and 13,000 domestic trips.

But should the El Salvador allegations hold up, it would put the entire culture of the service under even greater scrutiny.

"If these allegations are true, the behavior is absolutely unacceptable," Jackson Lee said.

She said Sullivan had begun putting together a 'working group that's going to deal with this issue of culture' and was restricting agent behavior.

"The agency will say no foreign nationals will be allowed in your room," the congresswoman said, and "it will be absolutely illegal in terms of your job for you to in essence attend or be associated with any place of bad acts."

The strip club owner made clear he hosted other US officials too, including embassy staff and FBI and Drug Enforcement Administration agents.

The State Department said on Thursday it will inquire with its embassy about the allegations.