The conflict over the territory of Nagorno-Karabakh, which lies inside Azerbaijan but has been under the control of ethnic Armenian forces since 1994, reignited on 27 September amid claims Turkey was sending militants to the region.
Dzens of people have already been killed, with Nagorno-Karabakh officials claiming that around 220 servicemen and 21 civilians have died in the clashes. Azerbaijani authorities have not confirmed any military casualties but reported the deaths of 25 civilians.
On Monday morning 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.
In turn the Azerbaijani Defense Ministry accused Armenian forces of shelling the towns of Tartar, Barda and Beylagan, and firing on Ganja, the country's second-largest city.
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 that the territory's forces would target military facilities in Azerbaijani cities in response to strikes on Stepanakert.
The Foreign Ministry of Iran, which shares a 470 mile-long border with Azerbaijan and a short border with Armenia, said it is working on a peace deal.
Ministry spokesman Saeed Khatibzadeh said: "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."
Mr Khatibzadeh also warned both sides against expanding the hostilities into Iranian territory, following reports that stray mortar shells had injured a child and damaged some buildings in rural areas in northern Iran.
"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.
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.
Azerbaijan's president has repeatedly said that the fighting will end only with Armenia's withdrawal from Nagorno-Karabakh.
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.
NATO Secretary-General Jens Stoltenberg met with the Turkish president Recep Tayyip Erdogan on Monday night to discuss the "military de-confliction mechanism" developed by the organisation for the Eastern Mediterranean.
He told reporters that NATO was "deeply concerned by the escalation of hostilities," and urged Turkey to "use its considerable influence to calm tensions."
Additional reporting by agencies