In a rare move, some Secret Service agents sent to Colombia to help protect U.S. President Barack Obama at a regional summit have been sent home amid accusations of a sex scandal, AFP reported.

"There have been allegations of misconduct made against Secret Service personnel in Cartagena, Colombia prior to the president's trip," Secret Service special agent in charge Edwin Donovan said in a statement. "Because of this, those personnel are being relieved of their assignments, returned to their place of duty, and are being replaced by other Secret Service personnel."

He did not specify what allegations had been made against the Secret Service staff, who were in Cartagena ahead of Obama's arrival late on Friday, April 13 for the Summit of the Americas which opens on Saturday.

But at least one of the agents had been involved with prostitutes in the Colombian resort city, the Washington Post said quoting Jon Adler, president of the Federal Law Enforcement Officers Association.

News of the scandal broke just as four home-made explosive devices went off, two in Cartagena and two in the Colombian capital Bogota late Friday near the U.S. embassy without causing any casualties or damage, police said.

But Donovan stressed the Secret Service staffing changes "will not affect the comprehensive security plan that has been prepared in advance of the president's trip."