Armenia accused Azerbaijan of firing missiles into the capital of the separatist territory of Nagorno-Karabakh on Monday, while Azerbaijan said several of its towns and its second-largest city were attacked.

Iran, which borders both countries, said it was working on a peace plan for the decades-old conflict, which reignited last month and has killed scores of people on both sides.

The region of Nagorno-Karabakh lies inside Azerbaijan but has been under the control of ethnic Armenian forces backed by Armenia since the end of a separatist war in 1994.

Armenian military officials reported missile strikes in the territorial capital of Stepanakert, which came under intense attacks all weekend. Residents told the Russian state RIA Novosti news agency that parts of the city were suffering shortages of electricity and gas after the strikes.

The Azerbaijani Defense Ministry, in turn, accused Armenian forces of shelling the towns of Tartar, Barda and Beylagan. Ganja, the country’s second-largest city far outside the conflict zone, also was “under fire,” officials said.

Hikmet Hajiyev, aide to Azeirbaijani President Ilham Aliyev, tweeted that Armenian forces attacked “densely populated civilian areas” in Ganja, Barda, Beylagan and other towns “with missiles and rockets.”

Armenia’s Foreign Ministry dismissed allegations of attacks being launched from Armenia’s territory as a “disinformation campaign” by Azerbaijan. Nagorno-Karabakh officials didn’t comment on the accusations, but warned on both Sunday and Monday that the territory’s forces would target military facilities in Azerbaijani cities in response to strikes on Stepanakert.

U.N. Secretary-General Antonio Guterres condemned the escalating violence and again urged an immediate halt to hostilities, stressing that there is no military solution to the conflict, his spokesman said.

The U.N. chief “is gravely concerned by reports of the extension of hostilities, including the targeting of populated areas,” spokesman Stephane Dujarric said, He urges a return to negotiations led by Russia, France and the United States — co-chairs of the so-called Minsk Group, which was set up by the Organization for Security and Cooperation in Europe in 1992 to resolve the conflict.

The fighting erupted Sept. 27 and has killed dozens, marking the biggest escalation in the conflict. Both sides have accused each other of expanding the hostilities beyond Nagorno-Karabakh.

According to Nagorno-Karabakh officials, about 220 servicemen on their side have died in the clashes since then. The state-run Armenian Unified Infocenter said that 21 civilians have been killed in the region and 82 others wounded.

Azerbaijani authorities haven’t given details about military casualties, but said 25 civilians were killed and 127 wounded.

Both sides have repeatedly accused each other of targeting civilians and have reported damage to nonmilitary infrastructure.

Azerbaijani President Aliyev said his troops “liberated” several more villages in the Jabrayil region. A similar report about the town of Jabrayil and its surrounding villages on Sunday was denied by Nagorno-Karabakh officials.

Nagorno-Karabakh was a designated autonomous region within Azerbaijan during the Soviet era. It claimed independence from Azerbaijan in 1991, about three months before the Soviet Union’s collapse. A full-scale war that broke out in 1992 killed an estimated 30,000 people.

By the time the war ended in 1994, Armenian forces not only held Nagorno-Karabakh itself but also substantial areas outside the territory borders, like the Jabrayil region where Azerbaijan claimed to have taken a town and several villages.

Aliyev has repeatedly said Armenia’s withdrawal from Nagorno-Karabakh is the sole condition to end the fighting.

Armenian officials allege Turkey is involved in the conflict on the side of Azerbaijan and is sending fighters from Syria to the region. Armenian Prime Minister Nikol Pashinian said “a cease-fire can be established only if Turkey is removed from the South Caucasus.”

Turkey, a NATO member, has denied sending arms or foreign fighters, while publicly siding with Azerbaijan.

Turkish President Recep Tayyip Erdogan reiterated that Turkey will stand with its ally Azerbaijan until it reaches “victory.” He also maintained that it was the international community’s silence in the face of what he called past Armenian aggression that encouraged it to attack Azerbaijani territory.

After talks with Turkish Foreign Minister Mevlut Cavusoglu in Ankara, NATO Secretary-General Jens Stoltenberg told reporters that the military alliance is “deeply concerned by the escalation of hostilities,” and urged Turkey to “use its considerable influence to calm tensions.”

Cavusoglu repeated calls for Armenia to withdraw from the region “in line with international laws, U.N. Security Council resolutions and Azerbaijan’s territorial and border integrity.”

The Foreign Ministry of Iran, which has nearly 760 kilometers (470 miles) of border with Azerbaijan and a short border with Armenia, said it is working on a peace plan.

Ministry spokesman Saeed Khatibzadeh did not elaborate but said Iran is talking to all related parties.

“Iran has prepared a plan with a specific framework containing details after consultations with both sides of the dispute, Azerbaijan and Armenia, as well as regional states and neighbors, and will pursue this plan,” he said.

Khatibzadeh also warned both sides against expanding the hostilities into Iranian territory.

“Any aggression against the borders of the Islamic Republic, even inadvertently, is a very serious red line for the Islamic Republic that should not be crossed,” he said.

Since the beginning of the conflict, stray mortar shells have injured a child and damaged some buildings in rural areas in northern Iran, near the border with Azerbaijan.