Russian President Vladimir Putin has called for an end to fighting after clashes broke out in the disputed Caucasus region of Nagorno-Karabakh.
Azerbaijan said 12 of its soldiers had been killed and a helicopter shot down. TV pictures showed a burned out vehicle and craters after gunfire and shelling.
Azerbaijan and Armenia accused each other of starting the fighting.
Nagorno-Karabakh has been in the hands of ethnic Armenian separatists since a war that ended in 1994.
Azerbaijan said its armed forces had came under fire first from large-calibre artillery and grenade-launchers, and that it had responded by destroying Armenian military hardware.
The Armenian government said Azerbaijan had started the fighting with tanks, artillery, and helicopters.
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.
Report says Defence Minister Sergei Shoigu has spoken with his Armenian and Azeri counterparts – Seyran Ohanyan and Zakir Hasanov by phone.
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.