Armenia and Azerbaijan vowed to keep fighting and rejected international calls for negotiations on Wednesday as clashes over the disputed Nagorny Karabakh region raged for a fourth day.

Armenian and Azerbaijani forces are engaged in the heaviest fighting in years over Karabakh, an ethnic Armenian province that broke away from Azerbaijan in the 1990s during the collapse of the Soviet Union.

The long-simmering conflict erupted on Sunday with the two sides trading heavy fire and blaming each other for the outbreak of violence.

The confirmed death toll surpassed 100 people including civilians Wednesday and both sides are claiming to have inflicted heavy losses on opposing forces.

In the Armenian capital Yerevan, dozens of men -- some already wearing military fatigues -- lined up outside a recruitment office to join the fight.

"We must act to defend our homeland against the aggressor," said Kamo, a 32-year-old factory worker. "This is our land. We will die before we abandon it."

Azerbaijan's President Ilham Aliyev promised his military would keep fighting until Armenian troops withdraw fully from Karabakh.

- No let-up in fighting -

If "the Armenian government fulfils the demand, fighting and bloodshed will end, and peace will be established in the region," he said during a visit with wounded soldiers.

Baku and Yerevan have ignored mounting international pressure for a ceasefire, as fears grow that the conflict could escalate into all-out war and draw in regional powers like Turkey and Russia.

The Armenian defence ministry on Wednesday accused Turkish aircraft of performing "provocative flights" along their shared border and of violating Armenia's airspace, a day after Yerevan said a Turkish jet had downed one of its warplanes.

Moscow, which has a military pact with Armenia but also good ties with Azerbaijan, has repeatedly called for an end to the fighting and offered to help with negotiations.

But Armenian Prime Minister Nikol Pashinyan said talks with Azerbaijan were not yet on the table.

"It isn't very appropriate to speak of a summit between Armenia, Azerbaijan and Russia at a time of intensive hostilities," Pashinyan said.

He said that Yerevan "at this point" is not planning to ask for intervention by a Russia-led military alliance, the Collective Security Treaty Organization that comprises several former Soviet republics including Armenia.

There has been no let-up in the fighting since the weekend, with both sides reporting new civilian casualties Wednesday.

- Burial for dead soldier -

Officials in both countries have made claims of huge losses for the other side, but these have not been possible to verify.

Azerbaijan has released no information on its military toll, but an AFP journalist in the southern Beylagan region saw dozens of women wailing over the coffin of a killed soldier, before men with Azerbaijan flags recited prayers at a burial.

The Armenian side has recorded 81 military deaths and 22 civilians -- eight Armenians and 14 Azerbaijanis -- have been reported dead.

Azerbaijan's defence ministry claimed Wednesday its forces have killed 2,300 Karabakh separatist troops since hostilities broke out.

The ministry said its troops had "destroyed 130 tanks, 200 artillery units, 25 anti-aircraft units, five ammunition depots, 50 anti-tank units, 55 military vehicles".

Karabakh's defence ministry, for its part, said Azerbaijani forces "continued artillery shelling" positions along the frontline.

The two sides have accused each other of targeting civilian areas, including in areas away from Karabakh.

Yerevan is claiming that Turkey, a longstanding ally of Azerbaijan, is providing direct military support, including mercenaries, for Baku.

It said on Tuesday that a Turkish F-16 flying in support of Baku's forces had downed an Armenian SU-25 warplane, but Ankara and Baku denied the claim.

- 'Close to large-scale war' -

"We are definitely very close to seeing a large-scale war, possibly even on a regional scale," Olesya Vartanyan of the International Crisis Group told AFP.

"If we see mass civilian casualties... that will be a very strong pretext for any regional power -- no matter Russia or Turkey -- to intervene," she said.

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 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.