Yerevan (AFP) - Four Armenian separatist fighters were killed Wednesday by Azerbaijani fire in the breakaway Nagorno-Karabakh region, rebels said, a day after US-mediated talks got under way between the historic rivals.

The clashes erupted after Washington opened three days of talks between Armenian and Azerbaijani officials Tuesday, in the latest attempt to quell a decades-long conflict that has flared repeatedly between the Caucasus foes.

The United States said it deplored the violence, but that the talks would continue.

Russia, once a security guarantor in the restive ex-Soviet region, said it was concerned by the fresh fighting.

“Units of the Azerbaijani armed forces opened fire with artillery” on Armenian positions on Wednesday morning, Karabakh’s defence ministry said.

“Four servicemen were killed in action as a result of another provocation by Azerbaijan,” it added, saying later that the situation was “relatively stable”.

Armenian Prime Minister Nikol Pashinyan warned of a “high risk of destabilisation”.

“I urge (the) international community to take practical steps to ensure (the) rights and security of Nagorno-Karabakh people,” he said on Twitter.

A US State Department spokesman, Vedant Patel, told reporters that: “We are deeply disturbed by the loss of life in Nagorno-Karabakh and we offer our condolences to the families of all of those who were killed.”

But negotiations begun on Tuesday “were constructive, and we continue to build on those discussions today and tomorrow, so there’s no change in the schedule,” he said.

Russia’s foreign ministry spokeswoman, Maria Zakharova, said Moscow was “concerned over the increasingly frequent armed incidents and ceasefire violations in Nagorno-Karabakh” and called for restraint from both sides.

- Frequent clashes -

There have been frequent clashes along the Armenian-Azerbaijani border, despite attempts by the two former Soviet republics to negotiate a peace agreement under mediation by the European Union and United States.

Azerbaijan’s defence ministry said Tuesday that “illegal Armenian armed detachments in the territory of Azerbaijan” had opened fire at army positions, wounding one Azerbaijani serviceman.

The same day, US Secretary of State Antony Blinken initiated closed-door negotiations with Armenian Foreign Minister Ararat Mirzoyan and his Azerbaijani counterpart Jeyhun Bayramov just outside Washington – the second round of such talks he has led in as many months.

Earlier this month, Pashinyan accused Baku of pursuing a policy of “ethnic cleansing” in Karabakh, saying traffic was being blocked through the Lachin corridor, the sole road linking the territory with Armenia.

The blockade followed a months-long roadblock by Azerbaijani environmental activists that Yerevan says has led to a humanitarian crisis in the mountainous enclave.

- Karabakh blockade claims -

Azerbaijan has insisted that civilians and aid convoys can travel through.

But the International Committee of the Red Cross said last week that Azerbaijan had now blocked access for convoys delivering aid to Karabakh, raising fresh concerns over shortages of food and medicine.

Russia has been seen as the main mediator between the two former Soviet republics, but Brussels and Washington have been increasingly active, with Moscow bogged down in Ukraine.

Armenia has repeatedly accused Russian peacekeepers of failing to live up to promises to protect ethnic Armenians in line with a 2020 truce, negotiated by Moscow after six weeks of fighting left thousands dead.

The ceasefire agreement saw Armenia cede swathes of territories it had controlled for decades.

When the Soviet Union collapsed in 1991, ethnic Armenian separatists in Karabakh broke away from Azerbaijan. The ensuing conflict claimed some 30,000 lives.