Four days of fighting in Nagorny Karabakh in early April killed at least 110 people as a festering conflict over the territory flared into the worst violence since a 1994 ceasefire that halted a brutal war.

Russian Foreign Minister Sergei Lavrov and US Secretary of State John Kerry are to hold talks Monday in Vienna with Azerbaijani President Ilham Aliyev and his Armenian counterpart Serzh Sarkisian, in a bid to defuse tensions between the two former Soviet neighbours.

A truce hammered out by Moscow halted the latest bloodshed but the situation remains on a knife-edge, with both sides accusing the other of violating the agreement.

"For now, the main aim for the mediators is to just calm down the tensions along the frontline," Armenia-based political analyst Hrant Melik-Shahnazaryan told AFP.

"The signing of any documents or reaching of any other sort of agreements is highly unlikely."

Azerbaijan and Armenia have been locked in a feud over Nagorny Karabakh since Armenian separatists seized the landlocked territory from Azerbaijan in a war that claimed some 30,000 lives in the early 1990s.

Since the 1994 ceasefire, the two sides have regularly exchanged fire across the frontline, but last month's violence was unprecedented since the shaky truce took effect.

With peace efforts spearheaded by Russia, the US and France stuttering to a halt in recent years, both sides in the conflict began rearming heavily, with energy-rich Azerbaijan especially spending vast sums on new weaponry.

And yet, despite increasingly feverish rhetoric from the rivals, the recent flare-up still appeared to catch the international community by surprise.

- 'Patriotic surge' -

While the two sides accused each other of starting the fighting, analysts said it seemed Azerbaijan -- which has been rocked by an economic crisis caused by falling oil prices -- launched the initial attack.

In the first shift in the frontline since 1994, Azeri forces seized several strategic positions, some of which they managed to cling on to despite a fierce Armenian counterattack.

"There was a patriotic upsurge of euphoria throughout the country, a useful distraction from the socio-economic crisis," Thomas de Waal of the Carnegie Europe think tank wrote in an op-ed for the Politico news site.

"The temptation -- and, worryingly, the public pressure -- to try this kind of offensive again is enormous," he added.

The agreement by the two leaders to hold direct talks in Vienna appears a positive sign, but few expect there to be any major progress at Monday's encounter.

"The two presidents want very different things. The Azerbaijani side wants new negotiations and to use its military force as leverage," de Waal wrote.

"The Armenian side is digging in harder -- they are reluctant to agree to anything that might look like submission to Azerbaijani military pressure and have demanded 'security guarantees'".

Moscow, which has sold weapons to both sides but has a military treaty with its close ally Armenia, is seen as central to stopping a conflict that some fear could end up spilling across the region.

Turkey, which has been at loggerheads with Moscow since Ankara downed a Russian jet near its border with Syria last year, has pledged to support its ally Azerbaijan.

Despite the international pressure being applied now to try to calm tensions, commentators on both sides feel that unless there is a conclusive resolution of the conflict there will be more eruptions of violence.

"If the meeting in Vienna does not yield any results, then the likelihood of a repeat of April fighting increases," said Elkhan Shainoglu of the Atlas think tank in Azerbaijan's capital Baku.