The top United Nations human rights official has condemned the reported killings of 11 Colombian lawmakers held hostage since 2002 by the nation's largest rebel group.

U.N. High Commissioner for Human Rights Louise Arbour Friday called for a full and impartial investigation into the killings. She said those responsible should be brought to justice according to international standards.

The Revolutionary Armed Forces of Colombia, or FARC, announced the deaths Thursday, saying the hostages were killed in a foiled military rescue mission on June 18.

But Colombian President Alvaro Uribe accused the rebels of executing the hostages, saying the Colombian military made no rescue attempt. He called the killings a crime against humanity.

The U.S. State Department called on the rebels to free all their captives, including three Americans.

The 11 hostages reported killed were abducted in April 2002 from the Valle del Cauca departmental Assembly. A 12th lawmaker abducted with them may have survived.

The human rights group Amnesty International is calling on the FARC and a smaller rebel group, the National Liberation Army (ELN), to immediately and unconditionally release all civilian hostages. Amnesty also wants the rebel groups to order their members to assure the humane treatment of those they have captured.

The rights group describes hostage-taking as "a flagrant breach of international humanitarian law, which can constitute a war crime."

Some information for this report was provided by AP .