Around thirty people have been killed in clashes between Azerbaijan and Armenia in the disputed Caucasus region of Nagorno-Karabakh.
Armenia said 18 ethnic-Armenian soldiers had died in the fighting, 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.
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.
Frozen conflict over Nagorno-Karabakh
-The conflict has roots dating back over a century to competition between Christian Armenian and Muslim Turkic and Persian influences
-Frictions exploded into violence when the region's parliament voted to join Armenia in the late 1980s
-The ethnic Azeri population - about 25% of the total before the war - fled Karabakh and Armenia while ethnic Armenians fled the rest of Azerbaijan
-Russian-brokered ceasefire signed in 1994, leaving Karabakh and swathes of Azeri territory around the enclave in Armenian hands
-Progress on a peace process stalled after talks between Armenian and Azeri leaders in 2009. Serious ceasefire violations have followed
-Karabakh is a word of Turkic and Persian origin meaning "black garden", while "Nagorno" is a Russian word meaning "mountain"