TEHRAN (Iran News) – A truce in Nagorno-Karabakh was under severe strain on Tuesday after fresh clashes between Azerbaijani and ethnic Armenian forces, as the rivals extended the deadliest fighting to hit the region since the 1990s.

A second attempt at a truce agreed on Saturday has had little impact on fighting that began on September 27, despite concerns that the fight could erupt into an all-out war involving Russia and Turkey, Al Jazeera reported.

Azerbaijan’s Ministry of Defense reported fighting in several areas, including disputed territory close to the line of contact that divides the sides.

Ethnic Armenian officials in Nagorno-Karabakh also reported fresh shelling on Tuesday and said fighting was particularly intense in southern areas of the conflict zone.

Nagorno-Karabakh is a breakaway region inside Azerbaijan, but run by ethnic Armenians who either want to secede or join Armenia.

A mountainous enclave, it lies close to gas and oil pipelines in the South Caucasus that go westward.

In less than one month of fighting, several hundred people have been killed, including dozens of civilians on both sides.

Ethnic Armenian officials say at least 772 troops have died, while Azerbaijan does not disclose its military death toll.

In a tweet on Tuesday, Azerbaijani President Ilham Aliyev said his country has made territorial gains.

However, Armenia, which has a defense pact with Russia, said it had repulsed Azerbaijani attacks and has the situation under control.

Al Jazeera was unable to independently verify either side’s claims.

In an interview with the BBC, Armenia’s Foreign Minister Zohrab Mnatsakanyan said the country was facing three enemies – Turkey, Azerbaijan and “foreign terrorists” – who he alleged were brought from Syria.

Armenian Prime Minister Nikol Pashinyan and Aliyev told Russia’s TASS news agency in separate interviews on Monday they were ready to visit Moscow for talks.

The new truce was meant to come into effect at the weekend and followed a similar deal brokered by Russia a week earlier.

In both instances, Armenia and Azerbaijan accused one another of breaking the truce within hours of agreed deadlines.

Meanwhile, according to reports, the Armenian and Azerbaijani foreign ministers are expected to hold talks with US Secretary of State Mike Pompeo in Washington, D.C. in separate meetings.

The Department of State did not immediately comment.

With fighting having spilled into a fourth week, international organizations have once again raised calls for the two sides to lay down their arms.

The United Nations Security Council members said on Monday Armenia and Azerbaijan should respect the new humanitarian truce agreement.

“Everyone was saying the same thing: The situation is bad and both sides need to pull back and heed the secretary-general’s calls for a cease-fire,” a UN diplomat told AFP.

On the possibility of sending UN observers, Vasily Nebenzya, Russia’s permanent representative to the UN, said it would require a mandate from the Security Council.

“This is not a quick process,” Nebenzya said.

He suggested any observer mission might also involve the Organization for Security and Cooperation in Europe (OSCE). The security watchdog has led mediation in the conflict for years through a panel known as the Minsk Group, headed by France, Russia and the US.

“Who will be there, in what capacity – is still a question that is being discussed,” said Nebenzya.

Azerbaijan’s ally Turkey has said the Minsk Group’s mediation has failed, while Baku has indicated it wants Ankara to be involved in peacemaking talks.