Dozens of people have been killed in clashes between Azerbaijan and Armenia in the disputed Caucasus region of Nagorno-Karabakh.
Armenia said 18 ethnic-Armenian troops died in the fighting early on Saturday, among the worst in two decades.
Azerbaijan said it had lost 12 troops and there were unconfirmed reports of civilian deaths on both sides.
Nagorno-Karabakh has been in the hands of ethnic-Armenian separatists since a war that ended in 1994.
There was more firing across the frontline overnight into Sunday, the two countries said, but no deaths were reported.
Russia, which has sold arms to both sides, called for an immediate ceasefire and for both sides to exercise restraint.
Azerbaijan said its armed forces had come under fire first from large-calibre artillery and grenade-launchers, and that it had taken over two strategic hills and a village.
The Armenian government said Azerbaijan had launched a "massive attack" with tanks, artillery and helicopters.
The Armenian-backed defence ministry in Karabakh said a 12-year-old boy had been killed and two other children injured.
Russian Defence Minister Sergei Shoigu has spoken with his Armenian and Azeri counterparts - Seyran Ohanyan and Zakir Hasanov - by phone, Interfax reported, in an effort to calm the situation.
Fighting between the two sides began in the late 1980s and escalated into full-scale war in 1991 as the Soviet Union collapsed, killing about 30,000 people before a ceasefire in 1994.
The region, which lies inside Azerbaijan but is controlled by ethnic Armenians, has since run its own affairs with Armenian military and financial backing, but clashes break out on a regular basis.
