Vladimir Putin has called for an end to fighting between Armenia and Azerbaijan around the disputed territory of Nagorno-Karabakh, amid reports of civilians being killed in the violence.
Armenia says it has shot down two Azerbaijani helicopters and destroyed three tanks in clashes between the two countries.
The breakaway region's defence ministry also claimed it had destroyed four Azeri helicopters, 15 drones and 10 tanks during clashes early on Sunday.
The Kremlin said Russia's President Mr Putin and Armenian Prime Minister Nikol Pashinyan discussed the situation in a phone call on Sunday.
A statement said the conversation noted that it was important not to allow further escalation of the conflict, and emphasised the need to halt all military action.
Russian foreign minister Sergey Lavrov "is conducting intensive contacts in order to induce the parties to cease fire and start negotiations to stabilise the situation", a spokeswoman for the ministry said.
Meanwhile, Azerbaijan's general prosecution office said five members of the same family had been killed by shelling from Armenia's forces – the first civilian casualties from Baku.
Armenia's defence ministry said the latest conflict began with an Azerbaijani attack – while Azerbaijan said the Armenian side attacked first and that they had launched a counter-offensive.
Both Armenia and Nagorno-Karabakh declared martial law and military mobilisation. However, Azerbaijan's army said there was no need for them to do this as they are fully staffed.
The Armenian defence ministry has released video footage that shows the attack on the vehicles, it says.
It said its troops had acted in response to an attack on civilian targets including Nagorno-Karabakh's capital, Stepanakert.
"Our response will be proportionate, and the military-political leadership of Azerbaijan bears full responsibility for the situation," it said in a statement.
Both sides have reported civilian deaths.
Armenian prime minister Nikol Pashinyan said on Twitter: "We stay strong next to our army to protect our motherland from Azeri invasion."
Hikmet Hajiyev, senior adviser to Azerbaijan's president, Ilham Aliyev, accused Armenian forces of launching "deliberate and targeted" attacks.
Azerbaijan's defence ministry said Armenia had attacked civilian settlements and military positions along the "contact line" – a heavily-mined no man's land that separates the Armenian-backed forces from Azeri troops in the region.
It said some civilians had been killed "as a result of the intensive shelling" by Armenia, and that Azerbaijan had taken retaliatory measures.
The ministry said Azerbaijan's army launched a "counter-offensive operation along the entire front to suppress the combat activity of the armed forces of Armenia and ensure the safety of the civilian population".
Turkey has demanded Armenia cease "hostilities", saying it risks plunging the region "into fire", while France has urged both sides to stop fighting and restart dialogue.
The two countries have long been at odds over the breakaway region, a conflict that has flared up again in recent months.
Nagorno-Karabakh declared independence from Azerbaijan soon after the collapse of the Soviet Union in 1991.
Though a ceasefire was agreed in 1994, Azerbaijan and Armenia frequently accuse each other of attacks around the area and along the separate Azeri-Armenian frontier.
Analysis: Clashes could draw Russia and Turkey into direct conflict
by Deborah Haynes, foreign affairs editor
Clashes that broke out on Sunday between Armenia and Azerbaijan over the breakaway region of Nagorno-Karabakh had only been a matter of time given the change in rhetoric between the two sides.
Hopes two years ago of a peace agreement had been replaced by renewed hostilities, which already flared up into deadly exchanges in July.
But Matthew Bryza, a former US ambassador to Azerbaijan, believes this weekend's violence is more serious than anything that has been seen since the two former Soviet Republics agreed to end a war over the region in the 1990s with a ceasefire.
That is because Azerbaijani shelling reached the regional capital, Stepankert, striking a residential block of flats, according to Armenia.
Azerbaijan has also claimed its forces seized control of a number of villages.
This is denied by Nagorno-Karabakh, but if it turns out to be true then that might well prompt Armenia-backed forces from the region to retaliate in a further escalation.
The two big powers in this part of the world – Turkey and Russia – will not want to see another costly war, not least because of the impact it would have on the transit of global oil and gas through pipelines that cross the region.
But this is a time of heightened tension between Ankara and Moscow, making Nagorno-Karabakh another flash point that could yet draw them into direct conflict.
Turkey has said it stands by majority Muslim Azerbaijan, while Russia – a long-standing ally of mainly Christian Armenia – is striking a more balanced tone, speaking to all parties involved and urging an immediate ceasefire.