Armenia's president headed to Brussels for talks with the European Union and NATO military alliance on Wednesday in a renewed drive to end the heaviest fighting since the 1990s over the mountain enclave of Nagorno-Karabakh.
As part of a wide diplomatic push from major global powers, the Azeri and Armenian foreign ministers also flew to Moscow for talks with Russia, which has brokered two ceasefires but failed to end clashes killing hundreds of people in the last month.
The warring sides' foreign ministers then go to Washington for talks on Friday involving U.S. Secretary of State Mike Pompeo, which has raised hopes of a breakthrough.
Both sides reported new fighting on Wednesday in and around Nagorno-Karabakh, a breakaway territory within Azerbaijan that is populated and controlled by ethnic Armenians.
Azerbaijan denied one of its military planes was shot down.
The fighting has raised fears of a wider war in the South Caucasus drawing in Russia and Turkey, an ally of Azerbaijan, and increased concern about the security of pipelines in Azerbaijan that carry Azeri gas and oil to world markets.
Armenian President Armen Sarkissian was due to meet NATO Secretary General Jens Stoltenberg, EU foreign policy chief Josep Borrell and European Council President Charles Michel in Brussels, his office said.
"It is expected that the leadership of NATO and the European organisations will do everything possible to stop Azerbaijan's and NATO-member Turkey’s military actions, which will help bring to life the ceasefire agreements," it said.
Armenia, which has a defence pact with Russia, has accused Turkey of sending mercenaries to fight in the conflict.
Turkey has denied this, and says it has no direct role in the fighting. But its tough rhetoric has increased strains in relations with Russia and with Turkey's NATO allies, who have ignored Azeri calls for Ankara to have a peacemaking role.
Azerbaijan says Armenian forces are the aggressors in the conflict, and its main condition for ending fighting is an Armenian withdrawal from Nagorno-Karabakh. Armenia rules this out and accuses Azerbaijan of making a land grab.
Azeri Foreign Minister Jeyhun Bayramov flew to Moscow for "consultations with the Russian side", and Armenian Foreign Minister Zohrab Mnatsakanyan was due to meet Russian Foreign Minister Sergei Lavrov. It was unclear if the foes' ministers would meet directly.
Mediation has for decades been led by Russia, France and the United States under the auspices of the Organization for Security and Co-operation in Europe (OSCE) security and rights watchdog. Its so-called Minsk Group is also expected to be represented at Friday's talks in Washington.
In the latest flareups, Azerbaijan said an Armenian regiment had been "forced out of action" and that fighting continued in several areas near the line of contact in Nagorno-Karabakh.
Armenian defence ministry spokeswoman Shushan Stepanyan said the town of Martakert in Nagorno-Karabakh was under heavy fire.
The sides' reports are often contradictory and hard to verify. Each accuses the other of attacking civilian targets and aid groups fear a humanitarian crisis.
Azerbaijan says its forces, which are boosted by increased military spending in the past few years and are making heavy use of drones, have made territorial gains.
Nagorno-Karabakh says it has repeatedly repulsed attacks and that the situation is under control despite the worst bloodshed since violence broke out as the Soviet Union collapsed, resulting in a war that killed about 30,000 people.