Heavy fighting has erupted between regional rivals Armenia and Azerbaijan in the tense separatist region of Nagorno-Karabakh.
Media reports on Saturday quoted Azerbaijan officials as saying that 12 of its soldiers had been killed. Armenian President Serzh Sarkisian said in a televised statement that 18 Armenian soldiers had been killed. Both sides also reported civilian casualties.
Each side blamed the other for starting the fighting and violating a two-decade-old cease-fire. Similar violence was reported last month.
Azerbaijan also confirmed that one of its military helicopters had been shot down. There was no information on what happened to the crew.
U.S. Secretary of State John Kerry released a statement condemning the cease-fire violations "in the strongest terms" and urged both sides to show restraint.
He said the "unstable" situation showed why Armenia and Azerbaijan must immediately enter talks on a comprehensive settlement sponsored by the Organization for Security and Cooperation in Europe.
U.N. Secretary-General Ban Ki-moon said he was "deeply disturbed" by the fighting and also called for negotiations.
Other demands for restraint came from Russian President Vladimir Putin and German Foreign Minister Frank-Walter Steinmeier.
On Thursday, U.S. Vice President Joe Biden met separately with Azerbaijani President Ilham Aliyev and Armenia's Sarkisian on the sidelines of a nuclear summit in Washington to express concern about continued violence in the region.
Biden urged both leaders to engage in dialogue and emphasized the importance of a comprehensive settlement for the long-term stability, security and prosperity of Nagorno-Karabakh.
The Nagorno-Karabakh region is part of Azerbaijan but has been under control of Armenian forces since 1994, when the two sides ended their war over the territory. Despite years of negotiations, no formal settlement has been reached and the situation in the area remains volatile.
The two sides are separated by a demilitarized buffer zone, but each side has accused the other of violating it numerous times.