Officials say 12 agents accompanying Barack Obama to leaders' meeting in Colombia have been relieved of duties

US secret service agents sent to provide security for Barack Obama at a summit in Colombia have been relieved of their duties and sent home after allegations of personal misconduct.

A secret service spokesman would neither confirm nor deny an anonymous claim to the Associated Press that the misconduct involved prostitutes in Cartagena, where the Summit of the Americas meeting is being held.

A US official said 12 agents had been relieved of duty. The official was not authorised to discuss the matter and requested anonymity.

In a statement on Friday night, the secret service spokesman Edwin Donovan said other agents had taken the place of those sent back to the US. He added that the personnel changes would not affect the security plan prepared for the president's visit.

Donovan said the allegations of misconduct were related to activity before the president arrived in Cartagena on Friday afternoon.

The Washington Post reported that Jon Adler, president of the Federal Law Enforcement Officers Association, had said the accusations related to at least one agent having involvement with prostitutes in Cartagena. The association represents federal law enforcement officers, including the secret service.

Ronald Kessler, a former Post reporter and the author of a book about the secret service, told the Post that he had learned that 12 agents were involved, several of them married.

Three small explosions were reported in Colombia before the summit – two on an empty area of land near Cartagena's bus station and one near the US embassy in the capital, Bogota. Officials said there were no injuries or damage.

On Friday evening Obama attended a leaders' dinner at Cartagena's historic Spanish fortress.

He was due to attend meetings with regional leaders on Saturday and Sunday. More than 30 presidents and prime ministers are attending the summit. Obama is expected to be on the defensive over issues including drug legalisation, which his administration opposes, the US insistence on excluding Cuba from the conference, and his opposition to Argentina's claim to the British-controlled Falkland Islands, which is supported by most Latin American countries.