Air and artillery attacks between the two counties broke out around the separatist region of Nagorno-Karabakh, a mainly ethnic Armenian region inside Azerbaijan which declared independence in 1991. Both countries pointed fingers at each other, with Armenia claiming Azerbaijan carried out an attack while officials in Baku said they were responding to Armenian shelling.
Armenia's government later declared martial law and a total military mobilization after a similar action by authorities in Nagorno-Karabakh.
"Get ready to defend our sacred homeland," Armenian Prime Minister Nikol Pashinyan said in a statement.
The Armenian Defense Ministry said an air and artillery attack on civilian settlements, including the regional capital of Stepanakert, began around 8:10 a.m. local time.
“We stay strong next to our army to protect our motherland from Azeri invasion," Pashinyan wrote on Twitter.
Officials said that attack prompted a response from Armenian troops, which resulted in three Azerbaijani tanks being destroyed and the shooting down of two helicopters and three drones.
“Our response will be proportionate, and the military-political leadership of Azerbaijan bears full responsibility for the situation,” the defense ministry said in a statement echoed by the foreign ministry.
Video from the region also showed tanks heading through Stepanakert as martial law was declared.
Azerbaijan denied the Armenian defense ministry statement, telling Reuters it had “complete advantage over the enemy on the front”, and accused Armenian forces of launching “deliberate and targeted” attacks along the front line which prompted a counteroffensive.
While the country also denied claims that its helicopters and tanks had been destroyed, President Ilham Aliyev said in a televised address to the nation that “there are losses among the Azerbaijani forces and the civilian population as a result of the Armenian bombardment.”
“We defend our territory, our cause is right!” Aliyev, said in an address to the nation.
Both sides reported casualties.
Artur Sarkisian, deputy head of the Nagorno-Karabakh army, told the Associated Press that 16 people were killed and more than 100 wounded. It wasn't immediately clear if the figure included both soldiers and civilians. Earlier, the Armenian human rights ombudsman said a woman and child had been killed in the shelling.
Azerbaijan’s defense ministry said an unspecified number of Azeri civilians had been killed and six wounded.
The outbreak of fighting was harshly received in Turkey, a close ally of Azerbaijan.
Turkey’s ruling party spokesman Omer Celik tweeted: “We vehemently condemn Armenia’s attack on Azerbaijan. Armenia has once against committed a provocation, ignoring law.” He promised Turkey would stand by Azerbaijan and said, “Armenia is playing with fire and endangering regional peace.”
Turkish presidential spokesman Ibrahim Kalin also took to Twitter to condemn Armenia. “Armenia has violated the ceasefire by attacking civilian settlements ... the international community must immediately say stop to this dangerous provocation.”
The recent conflict also puts the NATO ally up against Russia. Turkey has thrown its support behind Azerbaijan while Russia is a treaty ally of Armenia.
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.
It still remains unclear what sparked this latest round of fighting.
Nagorno-Karabakh is an ethnic Armenian enclave within Azerbaijan that has been out of Azerbaijan’s control since the end of a war in 1994. The people of Nagorno-Karabakh voted to secede from Azerbaijan and received Armenian military backing to do so.
Both sides have a heavy military presence along a demilitarized zone separating the region from the rest of Azerbaijan. The mostly mountainous region is about the size of Delaware and lies 30 miles from the Armenian border.
Local soldiers backed by Armenia also occupy some Azerbaijani territory outside the region. International efforts to settle the conflict have stalled and fighting sporadically breaks out.
Elin Suleymanov, the Azerbaijan ambassador to the U.S., told Fox News last year that the unresolved conflict was "not sustainable" and that conflict could "arise at any time."
He added the conflict between Azerbaijan and Armenia was "very strategically dangerous."
In July, one of the most severe outbreaks of fighting in years left 16 people from both sides dead. Another flare-up of violence in April 2016 left at least 200 dead.
Fox News' Hollie McKay, Lucas Tomlinson, and The Associated Press contributed to this report.