Armenia said Tuesday that a Turkish fighter jet had shot down one of its warplanes during heavy fighting with Turkey's ally Azerbaijan, but Ankara fiercely denied the claim.

Direct Turkish military action against Armenia would mark a major escalation after three days of heavy fighting between Armenian and Azerbaijani forces over the breakaway region of Nagorny Karabakh.

And the UN Security Council called on both sides for an immediate end to the fighting.

The two sides have defied calls for a ceasefire over Karabakh -- an ethnic-Armenian enclave that broke from Azerbaijan in the 1990s -- and are both claiming to have inflicted heavy losses on opposing forces.

Ankara has backed Azerbaijan in the conflict and on Tuesday the Armenian defence ministry said a Turkish F-16 flying in support of Baku's forces had downed an Armenian SU-25 warplane.

Ministry spokeswoman Shushan Stepanyan said the Turkish jet was supporting Azerbaijani aviation bombing civilian settlements in Armenia when it shot down the Armenian plane, killing the pilot.

Turkish President Recep Tayyip Erdogan's top press aide called the claim "absolutely untrue".

"Armenia should withdraw from the territories under its occupation instead of resorting to cheap propaganda tricks," said the aide, Fahrettin Altun.

Azerbaijan's President Ilham Aliyev said: "There is not a shred of evidence of Turkey's participation in the conflict." Ankara was giving only moral support, he insisted.

- 'Serious losses' -
Armenia and Azerbaijan have been locked for decades in a territorial dispute over Karabakh and have blamed each other for sparking fierce clashes that erupted on Sunday and have since caused nearly 100 confirmed deaths.

Foreign powers including the United States and Russia have called for an immediate ceasefire and a return to negotiations over the future of Karabakh, talks that have been stalled for years.

On Tuesday, the UN Security Council called on both sides to "immediately stop fighting", according to a statement seen by AFP. 

Earlier, both Azerbaijan's Aliyev and Armenian Prime Minister Nikol Pashinyan had vowed to continue fighting, their armies claiming to have dealt heavy blows to enemy forces.

The Armenian defence ministry said separatist forces in Karabakh had repelled Azerbaijani attacks along the frontline and that "the enemy suffered serious losses in manpower".

It claimed Azerbaijan's military had lost 72 drones, seven helicopters, 137 tanks, a plane and 82 military vehicles since Sunday. 

In Baku, officials denied that Armenian-backed separatists had regained control of territory lost in Sunday's fighting.

Azerbaijan said its military had repelled an Armenian counter-attack and destroyed a motorised column, an artillery unit and, later, an entire motorised infantry regiment.

Tuesday evening, Russia President Vladimir Putin spoke by phone with Pashinyan at the request of the latter. The Russia leader stressed the "urgent need for a ceasefire" and for the crisis to be defused.

- 'Waiting for 25 years' - 
The fighting between majority-Muslim Azerbaijan and Christian Armenia has raised fears of a wider conflict involving regional powers Turkey and Russia.

Armenia is part of a military alliance of former Soviet states led by Moscow, and the Kremlin on Tuesday urged Turkey and the warring sides to pursue "a peaceful settlement of this conflict using political and diplomatic means".

But Erdogan's aide Altun said Ankara was "fully committed to helping Azerbaijan take back its occupied lands".

Azerbaijan has not reported military casualties, but the Armenian separatist government has released footage from the battlefield showing what it said were the remains of Azerbaijani soldiers.

Shaddin Rustamov, a 25-year-old Azerbaijani conscript departing for training in Baku, told AFP he was proud to serve his country's military. 

Reclaiming Karabakh is "something we've been waiting 25 years for. Hopefully this year will be the last," he said.

Armenian officials confirmed Tuesday the deaths of three more civilians, while Baku said civilian casualties on the Azerbaijani side had reached 11.

That brings the total confirmed deaths in the fighting to 97, including 80 separatist fighters -- who reduced an earlier death toll by four -- and 17 civilians.

Pashinyan acknowledged the scale of the destruction and loss of life in an interview with Russian broadcaster Rossiya 1. But he added: "We see this as an existential threat for our people."

- US urges return to talks -
In a statement adopted unanimously during emergency talks on the conflict, the UN Security Council said its 15 members "voiced support for the call by the Secretary General on the sides to immediately stop fighting, de-escalate tensions and return to meaningful negotiations without delay." 

The council members said they "strongly condemn the use of force and regret the loss of life and the toll on the civilian population" in the ethnic Armenian enclave, which broke from Azerbaijan in the 1990s.

The council affirmed its "full support" for the central role of the co-chairs of the Organization for Security and Cooperation in Europe's Minsk Group (the US, Russia and France), who have mediated peace efforts.

US Secretary of State Mike Pompeo had also called for an end to fighting and a return to negotiations "as quickly as possible".

Karabakh's declaration of independence from Azerbaijan sparked a war in the early 1990s that claimed 30,000 lives, but it is still not recognised as independent by any country, including Armenia.

Armenia and Karabakh declared martial law and military mobilisation Sunday, while Azerbaijan imposed military rule and a curfew in large cities.

Talks to resolve the conflict -- which emerged amid the 1991 collapse of the Soviet Union -- have largely stalled since a 1994 ceasefire agreement.

France, Russia and the United States have mediated peace efforts as the "Minsk Group", but the last big push for a peace deal collapsed in 2010.

UN Council calls for Karabakh clashes to end

The UN Security Council on Tuesday called on Armenian and Azerbaijani forces to "immediately stop fighting" over the breakaway region of Nagorny Karabakh, after three days of deadly clashes, according to a statement seen by AFP.

The council's 15 members "voiced support for the call by the Secretary General on the sides to immediately stop fighting, de-escalate tensions and return to meaningful negotiations without delay," they said in a statement adopted unanimously during emergency talks on the conflict.

The council members said they "strongly condemn the use of force and regret the loss of life and the toll on the civilian population" in the ethnic Armenian enclave, which broke from Azerbaijan in the 1990s.

In the short statement, the council expressed concern over "reports of large scale military actions along the Line of Contact" in the conflict zone.

The council affirmed its "full support" for the central role of the co-chairs of the Organization for Security and Cooperation in Europe's Minsk Group (the US, Russia and France), who have mediated peace efforts.

It urged all parties to work closely with the co-chairs "for an urgent resumption of dialogue without preconditions."

Diplomats said the text, which was approved at the end of a meeting that lasted about an hour, was in fact proposed by the three co-chairs of the OSCE's Minsk Group, facilitating its adoption.

Armenia and Azerbaijan have been locked for decades in a territorial dispute over Karabakh and have blamed each other for sparking fierce clashes that erupted on Sunday and have since caused nearly 100 confirmed deaths.

Deadly fighting had flared earlier this year, and in 2016.