French President Emmanuel Macron has urged his Azerbaijan counterpart Ilham Aliyev to "return to respecting the ceasefire" with Armenia after fresh border clashes erupted between arch-foes Baku and Yerevan.

During a phone call this evening, he "told President Aliyev (of) the urgency of putting an end to hostilities, and to return to respecting the ceasefire", the French presidency said in a statement, after Armenia and Azerbaijan reported nearly 100 troop deaths today.

US Secretary of State Antony Blinken has said he was concerned that Russia could try to "stir the pot" in the conflict between Azerbaijan and Armenia as the United States urged the two countries to show restraint.

Washington has urged both sides in the conflict to cease hostilities after fighting broke out near the two countries' border.

Nagorno-Karabakh, a mountainous region of the South Caucasus, is internationally recognised as part of Azerbaijan, but the local ethnic Armenian residents reject Baku's sovereignty over the region. It has been the source of fierce tensions between the two countries for decades.

"Whether Russia tries in some fashion to stir the pot, to create a distraction from Ukraine, is something we're always concerned about," Mr Blinken told reporters at an event in Indiana, adding that Russia could also use its influence in the region to help "calm the waters".

White House spokesman John Kirby said the United States remained deeply concerned about reports of attacks along the countries' border and urged the governments of both countries to re-establish direct lines of communications across diplomatic and military channels.

"We've actively engaged with both the Armenian and the Azerbaijani government to ... see what we can do to end this violence," Mr Kirby told reporters at the White House. "There can be no military solution to this conflict. We urge restraint from any further military hostilities."

Mr Blinken held separate calls overnight with Armenia's prime minister and Azerbaijan's president to express Washington's concerns, the State Department said.

Earlier today, Russia called on Armenia and Azerbaijan to cease hostilities and observe a ceasefire agreement, expressing "extreme concern" over the renewed fighting.

In a statement, Russia's foreign ministry said it had brokered a ceasefire at 7am Irish time and it expected both sides to fulfil the terms of the agreement.

"We express our extreme concern over the sharp aggravation of the situation in areas of the Armenian-Azerbaijani border," the foreign ministry said.

"We call on the sides to refrain from further escalation of the situation, exercise restrain and strictly observe the ceasefire."

Armenia's Prime Minister Nikol Pashinyan said in a speech to parliament that the intensity of fighting had reduced, but was still active in some areas. Azerbaijani media said the ceasefire broke down within 15 minutes.

In its statement, Russia said the dispute should be resolved "exclusively through political and diplomatic means".

Both sides blamed each other for the flare-up in hostilities overnight, which Mr Pashinyan said left 49 Armenian soldiers dead.

Moscow, an ally of Yerevan through the Collective Security Treaty Organisation (CSTO) military alliance, but which also retains close friendly relations with Baku, did not say who it saw as responsible for the latest escalation.