Armenian and Azerbaijani forces accused each other of attacks on their territory on Tuesday, as fighting over the separatist region of Nagorno-Karabakh continued for a third straight day following the resumption of violence in the decades-old conflict.

The renewed fighting prompted calls from around the world for a ceasefire, before hostilities escalate.

Azerbaijan's Defence Ministry said Armenian forces shelled the Dashkesan region in Azerbaijan. Armenian officials said Azerbaijani forces opened fire on a military unit in the Armenian town of Vardenis, setting a bus on fire and killing one civilian.

Armenia’s Foreign Ministry denied shelling the Dashkesan region and said the reports were laying the groundwork for Azerbaijan “expanding the geography of hostilities, including the aggression against the Republic of Armenia.”

Dozens were killed and wounded since fighting broke out on Sunday. The Nagorno-Karabakh Defence Ministry reported 84 servicemen killed. Azerbaijan President Ilham Aliyev said on Tuesday that 10 civilians were killed on its side, but he didn't detail the country's military casualties.

Nagorno-Karabakh lies within Azerbaijan but has been under the control of ethnic Armenian forces backed by the Armenian government since 1994 at the end of a separatist war following the breakup of the Soviet Union three years earlier.

The region in the Caucasus Mountains of about 4,400 square kilometres, or about the size of the US state of Delaware, is 50km (30 miles) from the Armenian border. Soldiers backed by Armenia also occupy some Azerbaijani territory outside the region.

German Chancellor Angela Merkel has pushed for “an immediate ceasefire and a return to the negotiating table” in phone calls with the leaders of both countries, her office said.

She told them the Organisation for Security and Cooperation in Europe offers an appropriate forum for talks and that the two countries’ neighbours “should contribute to the peaceful solution”, said her spokesman, Steffen Seibert.

US Secretary of State Mike Pompeo said during a visit to Greece that “both sides must stop the violence” and work “to return to substantive negotiations as quickly as possible”.

Turkey supports Azerbaijan in the conflict, with President Recep Tayyip Erdogan urging Armenia to withdraw immediately from the separatist region.

Turkish Foreign Minister Mevlut Cavusoglu said Turkey is “by Azerbaijan’s side on the field and at the [negotiating] table”.

Cavusoglu said the international community must defend Azerbaijan's territorial integrity in the same way it defended the integrity of Ukraine and Georgia.

“They are holding Azerbaijan, whose territories have been occupied, on an equal footing with Armenia. This is a wrong and unjust approach,” Cavusoglu said after a visit to Azerbaijan’s Embassy in Ankara.

Russia, which along with France and the United States co-chairs the Minsk group set up in 1992 to resolve the Nagorno-Karabakh conflict, urged every country to help facilitate a peaceful resolution of the conflict.

“We call on all countries, especially our partners such as Turkey, to do everything to convince the opposing parties to cease fire and return to peacefully resolving the conflict by politico-diplomatic means,” Kremlin spokesman Dmitry Peskov said on Tuesday.