The United States on Friday expressed "deep concern" about Azerbaijan's pardoning of army officer Ramil Safarov after he was transferred from a Hungarian jail, a move that prompted Armenia to break off diplomatic ties with Hungary.

"The United States is extremely troubled by the news that the president of Azerbaijan pardoned Azerbaijani army officer Ramil Safarov, who returned to Baku today following his transfer from Hungary," State Department spokesman Patrick Ventrell said in a statement.

Safarov, 35, had been serving a life sentence in a Hungarian jail for murdering Armenian officer Gurgen Margaryan during a 2004 NATO training event in Budapest, the capital of Hungary. He was given a life sentence in 2006 by a Hungarian court.

"We are expressing our deep concern to Azerbaijan regarding this action and seeking an explanation," Ventrell said, adding "We are also seeking further details from Hungary regarding the decision to transfer Mr. Safarov to Azerbaijan."

Armenia responded by cutting off diplomatic ties with Hungary, as the Central Asian nation is still in conflict with Azerbaijan over Nagorno-Karabakh, a region that lies within Azerbaijan but has been controlled by Armenian troops and ethnic forces since a separatist war broke out in 1988.

Armenia and Azerbaijan signed a cease-fire agreement in 1994 but the conflict has never been truly settled despite mediation by the Minsk Group of the Organization for Security and Cooperation in Europe that was co-chaired by France, Russia and the United States.

"We condemn any action that fuels regional tensions," said Ventrell.