Russian President Vladimir Putin has called for an end to fighting after clashes broke out in the disputed Caucasus region of Nagorno-Karabakh.
TV pictures showed a burned out vehicle and craters after a night of gunfire and heavy shelling.
Azerbaijan and Armenia accused each other of starting the fighting, with both sides reporting casualties.
Nagorno-Karabakh has been in the hands of ethnic Armenian separatists since a war that ended in 1994.
Azerbaijan said its armed forces came under fire first from large-calibre artillery and grenade-launchers. The Armenian government said Azerbaijan started the fighting with tanks, artillery, and helicopters.
The Nagorno-Karabakh military said Armenian anti-aircraft fire had shot down an Azeri helicopter, but Azerbaijan denied this and said it had destroyed military hardware and inflicted losses on the Armenian side.
Similar violence was reported last month.
Russia, which sells arms to both Armenia and Azerbaijan, has called for an immediate ceasefire and to exercise restraint.
Defence Minister Sergei Shoigu has spoken with his Armenian and Azeri counterparts - Seyran Ohanyan and Zakir Hasanov - by phone, Interfax reported.
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
- Russia-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"