YEREVAN, Armenia (AP) — Fighting between Armenian and Azerbaijani forces has erupted again over the disputed separatist region of Nagorno-Karabakh and the territory’s defense ministry said 16 soldiers and two civilians have been killed and more than 100 others wounded.

Azerbaijan’s president, meanwhile, says his military has suffered losses, but gave no details.

Armenia also claimed that four Azerbaijani helicopters were shot down and 33 Azerbaijani tanks and fighting vehicles were hit by artillery. Azerbaijan’s defense ministry rejected an earlier claim that two helicopters were shot down.

The heavy fighting broke out in the 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.

Nagorno-Karabakh authorities reported that shelling hit the region’s capital of Stepanakert and the towns of Martakert and Martuni. Armenian Defense Ministry spokesman Artsrun Hovhannisyan also said Azerbaijani shelling hit within Armenian territory near the town of Vardenis.

The territorial defense ministry said late Sunday that 18 people were killed, including a woman and her grandson and more than 100 wounded.

Azerbaijani President Ilham Aliyev ordered martial law be imposed in some regions of the country and called for a curfew in major cities.

In a televised address to the nation, Aliyev said that “there are losses among the Azerbaijani forces and the civilian population as a result of the Armenian bombardment,” but didn’t give further details. He also claimed that “many units of the enemy’s military equipment have been destroyed.”

Russian Foreign Minister Sergey Lavrov “is conducting intensive contacts in order to induce the parties to cease fire and start negotiations to stabilize the situation,” Foreign Ministry spokeswoman Maria Zakharova said.

Albanian Prime Minister Edi Rama, chairman of the Organization for Security and Cooperation in Europe, called on the sides to stop fighting. The long-unsuccessful negotiations for resolving the territory’s status has been conducted under OSCE auspices.

Armenian Prime Minister Nikol Pashinian on Sunday said the country could reexamine whether to recognize Nagorno-Karabakh as independent. Such a move would likely obstruct further negotiations.

Foreign Minister Javad Zarif of Iran, which borders both Azerbaijan and Armenia, said “We call for an immediate end to hostilities and urge dialogue to resolve differences. Our neighbors are our priority and we are ready to provide good offices to enable talks.”

Turkish presidential spokesman Ibrahim Kalin condemned Armenia.

“Armenia has violated the cease-fire by attacking civilian settlements … the international community must immediately say stop to this dangerous provocation,” Kalin tweeted. Turkey is a close ally of Azerbaijan and locked in a long dispute with Armenia that has closed the countries’ border since the early 1990s.

United Nations Secretary-General Antonio Guterres “calls on the sides to immediately stop fighting, de-escalate tensions and return to meaningful negotiations without delay,” said his spokesman Stephane Dujarric. U.S. Deputy Secretary of State Stephen Biegun called the foreign ministers of Armenia and Azerbaijan to urge cessation of hostilities, said State Department spokesperson Morgan Ortagus.

Mostly mountainous Nagorno-Karabakh — a region around 4,400 square kilometers (1,700 square miles) or about the size of the U.S. state of Delaware — lies 50 kilometers (30 miles) from the Armenian border. Local soldiers backed by Armenia also occupy some Azerbaijani territory outside the region.

Jim Heintz in Moscow, Zeynep Bilginsoy in Istanbul and Edith M. Lederer at the United Nations contributed to this story.