Clashes between Armenia and Azerbaijani forces over the disputed separatist region of Nagorno-Karabakh are continuing after hostilities broke out on Sunday.

The Azerbaijani Defence Ministry claimed that Armenian forces started shelling the town of Tartar on Monday morning, while Armenian officials said the fighting continued throughout the night and Baku resumed “offensive actions” in the morning.

Azerbaijan’s Defence Ministry told the Interfax news agency that more than 550 Armenian troops have been killed, a claim that Armenian officials denied.

According to Nagorno-Karabakh officials, 31 servicemen have been killed so far. Armenian Defence Ministry spokesman Artsrun Hovhannisyan said on Monday that over 200 people have been wounded.

The heavy fighting broke out on Sunday morning in the region that lies within Azerbaijan but has been under the control of ethnic Armenian forces backed by Armenia since 1994 at the end of a separatist war.

It was not immediately clear what sparked the fighting, the heaviest since clashes in July killed 16 people from both sides.

Mostly mountainous Nagorno-Karabakh – a region around 1,700 square miles — lies 30 miles from the Armenian border. Local soldiers backed by Armenia also occupy some Azerbaijani territory outside the region.