U.S. Homeland Security Secretary Janet Napolitano says Barack Obama's security was "never at risk" from a recent incident allegedly involving Secret Service agents and Colombian prostitutes that embarrassed the White House and overshadowed the president's visit to a Latin American summit.

Appearing before a Senate panel Wednesday, Napolitano promised a "complete and thorough" investigation into the "inexcusable" allegations facing a number of agents but added she would not allow for "the actions of a few to tarnish the proud legacy of the Secret Service."

"We will leave no stone unturned," she told the hearing.

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A dozen secret service agents sent to provide security for U.S. President Barack Obama in Cartagena, Colombia, were relieved from duty and replaced with other agency personnel after an incident of alleged misconduct. (Fernando Llano/Associated Press)

At the same time, she said there was "no impinging on the security of the president" from the alleged actions of the agents involved in the affair.

The Secret Service says two more of those implicated have been ousted, one stripped of his clearance and two cleared, resolving questions about the fates of the full dozen under investigation.

Napolitano also praised the professionalism and bravery shown by other members of the agency every day.

"I have nothing but respect for these men and women, many of whom put their lives at risk for protecting the president," she said.

Judiciary Committee Chairman Patrick Leahy told Napolitano in his remarks opening the hearing that the agency was currently entrusted with protecting the two likely candidates for president in November's election.

"No one wants to see America embarrassed," Leahy said. "I can't think of anything that would look worse to the rest of the world than if something were to happen to either President Obama or Governor Romney."

Other senators, particularly Republicans, had big plans for the televised hearing, the first public proceedings that touched on the scandal. The Secret Service announced late Tuesday that all 12 implicated had been dealt with: nine forced out, one stripped of his security clearance and two cleared of wrongdoing, all within two weeks of the night in question.

The scandal erupted after a fight over payment between a Colombian prostitute and a Secret Service employee spilled into the hallway of the Hotel Caribe ahead of President Barack Obama's arrival at the Summit of the Americas in Cartagena.

A dozen military personnel have also been implicated, and Defence Secretary Leon Panetta said they have had their security clearances suspended.

Obama blames 'knuckleheads', defends agency

Further stealing his critics' thunder, Obama said Tuesday the employees at the centre of the scandal were not representative of the agency that protects his family in the glare of public life.

"These guys are incredible. They protect me. They protect Michelle. They protect the girls. They protect our officials all around the world," the president said on NBC's Late Night With Jimmy Fallon.

"A couple of knuckleheads shouldn't detract from what they do," Obama added. "What these guys were thinking, I don't know. That's why they're not there anymore."

Some of the Judiciary Committee members still had questions. Ranking Republican Charles Grassley of Iowa, for example, wants to know whether Napolitano's inspector general has opened an independent investigation.

Lawmakers across Congress say they are concerned about the security risk posed by the proximity the prostitutes — as many as 20, all foreign nationals — had to personnel with sensitive information on the president's plans.