CHAPEL HILL, North Carolina — US President Barack Obama on Tuesday branded Secret Service agents embroiled in a Colombian prostitute scandal as "knuckleheads" who should not detract from the "incredible" work of the rest of his elite bodyguard unit.

"The Secret Service, these guys are incredible," Obama said at a taping of the NBC talk show "Late Night with Jimmy Fallon" due to air later on Tuesday.

"They protect me, they protect our girls. A couple of knuckleheads shouldn't detract from that they do. What they were thinking? I don't know. That's why they're not there anymore."

A total of six Secret Service agents have lost their jobs over the scandal involving prostitutes who they allegedly invited back to their hotel in Cartagena, Colombia during a trip to prepare security for Obama's visit to the Summit of the America's 10 days ago.

In all 12 Secret Service officers and 12 military personnel are being investigated over the affair, a huge diplomatic embarrassment for the United States. The White House said Monday that an internal probe had established that no White House civilian staff were involved in the sleazy goings on in the Caribbean resort.

Investigations by the Pentagon and the Secret Service are still taking place.