The capital of the disputed Nagorno-Karabakh region in Azerbaijan was shelled on Thursday, Armenian separatists in the province said, ending a lull in bombardments brought on by a ceasefire.

Emergency services in Karabakh wrote on Facebook that Azerbaijani forces struck "civilian targets" in Stepanakert, in what is the first confirmed strike on the city since Armenia and Azerbaijan agreed a truce in Moscow last Saturday.

An AFP journalist in Stepanakert reported hearing nine loud explosions and said smoke could be seen rising near residential buildings on the outskirts of the city.

Stepanakert has been shelled on and off since fighting between Armenian separatists and Azerbaijan's forces erupted late last month in clashes that have claimed hundreds of lives, including civilians.

But the situation in the city had been calm since the longstanding ex-Soviet rivals agreed to halt fighting last week to exchange prisoners and bodies of people killed in the fighting.

Armenia and Azerbaijan's foreign ministers agreed a humanitarian ceasefire, but both sides accused the other of violating the agreement within hours after it was supposed to take effect.

Azerbaijani forces have advanced along the north and south of the frontline, taking positions in Karabakh and in two districts of Azerbaijan that have been under Armenian control since 1994.

Armenian Prime Minister Nikol Pashinyan conceded Wednesday that separatist fighters had been forced to withdraw from positions in the north and south, describing the situation as "very serious".