U.S. co-chairman of the OSCE Minsk Group James Warlick is concerned about tensions along the Line of Contact of Azerbaijani and Armenian troops.
"Violence and tensions along the Line of Contact undermine efforts to bring about a negotiated settlement," Warlick wrote on Twitter.
The conflict between the two South Caucasus countries began in 1988 when Armenia made territorial claims against Azerbaijan.
Armenian armed forces have occupied 20 percent of Azerbaijan since 1992, including the Nagorno-Karabakh region and seven surrounding districts.
Azerbaijan and Armenia signed a ceasefire agreement in 1994. The co-chairs of the OSCE Minsk Group, Russia, France and the U.S. are currently holding peace negotiations.
Armenia has not yet implemented the U.N. Security Council's four resolutions on the liberation of the Nagorno-Karabakh and the surrounding regions.