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.
Analysis: Konul Khalilova, BBC Azeri
The fighting that erupted on Friday night is some of the worst since a 1994 ceasefire between the two sides. Azerbaijan says it has taken back two strategically important villages from the Armenian army, a claim denied by Armenia. As usual, both sides say the other pulled the trigger first.
There are reports of civilian casualties on both sides. Witnesses told the BBC's Azeri service that people were being evacuated from villages near to the conflict zone and that others were hiding in basements.
Both President Aliyev of Azerbaijan and Armenia's President Sargsyan are on their way back from the international nuclear summit in Washington.
Azerbaijan has purchased at least $4bn worth of arms from Russia. Armenia, an important strategic partner of Russia in the Caucasus, also buys weapons from Russia. There are concerns that the fighting could lead to a more wide-scale military conflict.
Leaders on both sides have been blamed for not making enough effort to achieve peace and instead using the conflict as a tool to stay in power. Nationalist sentiment boosted by pro-government media in both societies has been at its height in recent years.
The Organization for Security and Co-operation in Europe (OSCE) has expressed "grave concern" over the reported large-scale ceasefire violations.
The co-chairmen of the body's Minsk Group - ambassadors Igor Popov of Russia, James Warlick of the US, and Pierre Andrieu of France - issued a joint statement saying: "We strongly condemn the use of force and regret the senseless loss of life, including civilians.
"The co-chairs call upon the sides to stop shooting and take all necessary measures to stabilise the situation on the ground. They reiterate that there is no alternative to a peaceful negotiated solution of the conflict and that war is not an option."
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 an Azeri word meaning "black garden", while "Nagorno" is a Russian word meaning "mountain"