Colombian authorities have blamed leftist rebels for a bomb blast Wednesday that killed seven people and injured 68 others at a shopping center in the northeastern city of Cucuta.

Police said rebels of the National Liberation Army, the ELN, were responsible for the blast, which ripped through an underground parking lot. Authorities offered no evidence and the guerrillas have not claimed responsibility.

The ELN, as well as the nation's largest rebel group, the Revolutionary Armed Forces of Colombia, FARC, are heavily active in Cucuta.

In response to the bombing, Colombian President Alvaro Uribe on Wednesday fired two top security officials in the region, saying rebels have infiltrated government offices. The state police chief Orlando Vale Mosquera was removed, along with a director in the public prosecutor's office, Nestor Pacheco.

The attack occurred as Mr. Uribe met with U.S. Under Secretary of State Marc Grossman at the presidential palace in Bogota. The two leaders discussed rescue operations to find three Americans who were kidnapped by the FARC three weeks ago. The three were in a plane that crashed in southern Colombia.

A fourth American and a Colombian soldier on the plane were found shot to death at the crash site.

Mr. Grossman emphasized the United States would not negotiate with the rebels holding the hostages. He also called on Colombia's neighbors to help combat terrorist groups operating in the region.

Colombia has been torn by a 39-year civil war that pits the rebels, paramilitaries and the government against each other. Some 3,500 people, mainly civilians, are killed in the fighting each year.