France, Russia and the US have demanded an immediate ceasefire between Azerbaijan and ethnic Armenian forces around the Caucasus region of Nagorno-Karabakh, calling for a return to negotiations without delay.
"We call for an immediate cessation of hostilities between the relevant military forces," the French, Russia and US presidents said in a joint statement in their capacity as co-chairs of the OSCE Minsk Group.
"We also call on the leaders of Armenia and Azerbaijan to commit without delay to resuming substantive negotiations, in good faith and without preconditions, under the auspices of the OSCE Minsk Group Co-Chairs."
The group was set up in 1992 to mediate a peaceful resolution over the Nagorno-Karabakh enclave in the South Caucasus.
The call for immediate negotiations came as the death toll rose in the heaviest clashes around the Nagorno-Karabakh region since the 1990s.
The Kremlin said President Vladimir Putin and French President Emmanuel Macron had discussed steps that the Organisation for Security and Co-operation's (OSCE) Minsk group, which mediates in the conflict, could take to end the fighting.
Russia has also offered to host the foreign ministers of Armenia and Azerbaijan for talks on the flare-up of their decades-old conflict in the volatile South Caucasus region.
Azerbaijan's general prosecutor's office said Armenian shelling had killed a civilian in the Azeri town of Terter on Thursday morning and badly damaged the town's train station.
Armenia's defence ministry spokeswoman said the situation remained tense and Azerbaijan's forces had tried to regroup but been prevented from doing so.
Dozens have been reported killed and hundreds wounded since fighting broke out over Nagorno-Karabakh, a mountainous enclave inside Azerbaijan that is administered by ethnic Armenians and broke away in a 1991-94 war that killed 30,000.
The re-eruption of one of the "frozen conflicts" dating back to the collapse of the Soviet Union has raised concerns about stability in the South Caucasus, a corridor for pipelines carrying oil and gas to world markets, and raised fears that regional powers Russia and Turkey could be drawn in.
The Kremlin statement said there was no alternative to using "political and diplomatic methods" to resolve the crisis.
Macron's office said the two leaders "also shared their concern regarding the sending of Syrian mercenaries by Turkey to Nagorno-Karabakh".
The Kremlin statement made no mention of this. But Russia's foreign ministry said on Wednesday that Syrian and Libyan fighters from illegal armed groups were being sent to Nagorno-Karabakh.
NATO allies France and Turkey traded angry recriminations on Wednesday over the fighting in Nagorno-Karabakh, which is not internationally recognised as an independent republic.
Turkey, a close ally of mainly Muslim Azerbaijan, said it would "do what is necessary" to support Azerbaijan, and that French solidarity with Armenia amounted to supporting Armenian occupation in Azerbaijan.
Macron, whose country is home to about 600,000 people of Armenian origin, accused Turkey of "warlike" rhetoric.