Azerbaijan says it shelled Sotk in retaliation of Armenia’s attack on Kalbajar region that wounded two of its soldiers.

Four Armenian soldiers have been killed after Azerbaijani shelling near the border town of Sotk, northwest of the breakaway region of Nagorno-Karabakh according to Armenia’s Ministry of Defence.

Tensions between Baku and Yerevan have escalated sharply in recent months, as both sides accuse the other of violating agreements and cross-border gunfire.

“As a result of an Azerbaijani provocation, four servicemen were killed and one wounded on the Armenian side,” Armenia’s defence ministry said on Friday, after earlier reporting two were killed.

Meanwhile, Azerbaijan said that Armenia had struck positions in the Kalbajar region using drones, wounding three Azerbaijani servicemen. It said it was taking “retaliatory measures”.

Separately, a Azerbaijani soldier was also injured in cross-border fire.

“We declare that all responsibility for the tension and its consequences lies with the military-political leadership of Armenia,” Baku’s defence ministry said.

Nagorno-Karabakh, internationally recognised as part of Azerbaijan, has been a source of conflict between the two Caucasus neighbours since the years leading up to the collapse of the Soviet Union in 1991.

Yerevan and Baku have fought two wars for control over the region, which is internationally recognised as part of Azerbaijan but largely populated by ethnic Armenians.

The two sides have been unable to reach a lasting peace settlement despite mediation efforts by the European Union, United States and Russia.

Azerbaijan accused Armenia of building up troops along the two countries’ volatile borders in August, while Armenia accused Azerbaijan’s military of opening fire on European Union observers.

Separatist authorities in the breakaway region of Nagorno-Karabakh said in June that four Armenian soldiers were killed by Azerbaijani fire in Nagorno-Karabakh.