The attacks on Tuesday raised the threat of a new war in the ethnic Armenian region of Azerbaijan, which has been a flashpoint since the collapse of the Soviet Union. It is internationally recognised as Azerbaijani territory but part of it is run by separatist Armenian authorities who have said the area, with a population of about 120,000, is their ancestral homeland.

Baku launched what it has called an “anti-terrorist operation” hours after four soldiers and two civilians were killed by landmines that it claimed were planted by Armenian saboteurs.

Azerbaijan’s defence ministry said it was intended to “disarm and secure the withdrawal of formations of Armenia’s armed forces from our territories, [and] neutralise their military infrastructure”.

Azerbaijani forces on Tuesday seized more than 60 military posts and destroyed up to 20 military vehicles with other hardware, the ministry said in a statement.

Armenia’s foreign ministry condemned the attacks and said Azerbaijan had “unleashed another large-scale aggression against the people of Nagorno-Karabakh, aiming to complete its policy of ethnic cleansing”.

It was not immediately clear how many people were killed or injured as a result of the military offensive. A separatist Armenian human rights official in the breakaway region of Nagorno-Karabakh said 25 people had been killed, including two civilians. Al Jazeera was unable to verify the claim.

The foreign policy adviser to Azerbaijain President Ilham Aliyev, Hikmet Hajiyev, told Al Jazeera that Baku had launched “local but limited counter-terrorism measures” aimed at striking military targets.

He claimed that while Baku was using high-precision weapons, “collateral damage” was likely unavoidable as civilians were being used as “human shields” in the contested region.

“We call on all civilians to take a safe distance from military targets,” he said.

The state news agency quoted the presidential administration as saying that Azerbaijan would continue the operation “until the end” unless “Armenian military units” surrender and give up their weapons.

Nagorno-Karabakh and sizable surrounding territories came under the control of ethnic Armenian forces backed by the Armenian military at the 1994 end of a separatist war. Azerbaijan regained the territories and parts of Nagorno-Karabakh itself in fighting in 2020.

Armenia, which has said its armed forces are not in Karabakh and that the situation on its own border with Azerbaijan is stable, called on members of the UN Security Council to help and for Russian peacekeepers deployed since the end of the previous conflict in 2020 to intervene.

In the Armenian capital Yerevan, protesters gathered to denounce Prime Minister Nikol Pashinyan’s handling of the Karabakh crisis and demand his resignation.

The demonstrations took place after Pashinyan – viewed as too pro-Western by Russia, Armenia’s traditional supporter – denounced calls for a “coup” as Azerbaijan launched its military operation.

Armenia’s security council warned of “a real danger of mass turmoil in the Republic of Armenia” following the unrest.

Kremlin spokesperson Dmitry Peskov said Russia had contacted both parties urging renewed negotiations.

Moscow, which is waging its own war against neighbouring Ukraine, is seeking to preserve its influence in the face of greater activity from Turkey, which backs Azerbaijan.

Turkey’s foreign ministry defended Baku, saying Azerbaijan had been forced to take measures on its sovereign territory in Nargorno-Karabakh after its concerns were not alleviated following the 2020 conflict with Armenia.

“Whether Russia is now able to mediate a renewed ceasefire remains to be seen. This would probably come with a high political cost for the Armenian government,” Marie Dumoulin, director of the Wider Europe Programme at the European Council on Foreign Relations, told Al Jazeera in a statement.

Moscow’s peacekeeping troops deployed in the contested region after the 2020 ceasefire haven’t prevented any of the Azerbaijani military actions since then.

Their activity has been mostly blocked since December as Azerbaijan alleged that Armenians were smuggling in weapons and conducting illicit resource extraction. Armenia has said that Azerbaijan imposed a blockade on the territory that led to severe food shortages, and that Azerbaijan aimed for a genocide by starvation.

Armenia has accused Moscow of being too distracted by its own war in Ukraine to protect it and said Russian peacekeepers in Karabakh were failing to do their job.

Western leaders call for negotiations, end of hostilities

The United States said it was pursuing crisis diplomacy over what it believed was a particularly dangerous flare-up. US Secretary of State Antony Blinken was likely to get involved in the next 24 hours in trying to defuse the crisis, US officials said.

The European Union, France and Germany condemned Azerbaijan’s military action, calling for a return to talks on the future of the region.

The EU’s foreign policy chief Josep Borrell said Brussels remained “fully engaged” in facilitating the dialogue. “This military escalation should not be used as a pretext to force the exodus of the local population”, he said.

French President Emmanuel Macron urged “an immediate resumption of discussions” to find a “just and lasting peace” between Armenia and Azerbaijan, calling for an “immediate cessation of the offensive”, according to a news statement.

German Foreign Minister Annalena Baerbock said Azerbaijan had broken its promise not to resort to military action on Nagorno-Karabakh. “Azerbaijan must immediately stop shelling and return to the negotiating table,” Baerbock said on the sidelines of the United Nations General Assembly in New York.