Published2 hours ago
One of the world's oldest conflicts, a territorial dispute between Armenia and Azerbaijan, has re-erupted into fierce combat that killed at least 23 people.
Helicopters were reportedly shot down and tanks destroyed as the two ex-Soviet republics battled over the breakaway Nagorno-Karabakh region.
The region is internationally recognised as part of Azerbaijan but controlled by ethnic Armenians.
When it broke away in the early 1990s, tens of thousands died in fighting.
Ethnic and religious differences which had been suppressed under communism resurfaced with devastating consequences.
Azerbaijani President Ilham Aliyev said on Sunday he was confident of regaining control over the breakaway region.
Martial law has been declared in some regions of Azerbaijan, as well as in Armenia and Nagorno-Karabakh.
The conflict in the Caucasus Mountains has remained unresolved for more than three decades, with periodic bouts of fighting.
Border clashes in July killed at least 16 people, prompting the largest demonstration for years in the Azerbaijani capital Baku, where there were calls for the region's recapture.
Any upsurge could unsettle markets as the South Caucasus is a corridor for pipelines carrying oil and natural gas from the Caspian Sea to world markets.
Turkey's President Recep Tayyip Erdogan pledged support for Azerbaijan during the new crisis while Russia, traditionally seen as an ally of Armenia, called for an immediate ceasefire and talks to stabilise the situation.
Emboldened by Turkish support?
By Rayhan Demytrie, BBC Caucasus correspondent
Sunday's fighting with the use of heavy weaponry along the line of control is the most serious escalation in recent years.
It is common in this decades-long conflict for both sides to accuse the other of firing the first shots and what we are seeing is not just military action but also an information war. It is difficult to independently verify official information.
Azerbaijan's claim to have "liberated" territory controlled by Armenians has been denied by the Armenian authorities. Similarly, Armenia's claims to have inflicted heavy losses on Azerbaijani forces have been dismissed by Baku. Moreover, the Azerbaijani authorities have restricted internet use inside the country, in particular access to social media.
Turkey's emphatic support may embolden Azerbaijan. Back in August, the Azerbaijani defence minister said that with the help of the Turkish military Azerbaijan would fulfil its "sacred duty" - in other words, take back its lost territories.
How did the fighting spread?
Armenia's defence ministry said an attack on civilian settlements in Nagorno-Karabakh, including the regional capital Stepanakert, began at 08:10 local time (04:10 GMT) on Sunday.
A woman and child were killed, officials said. The separatist authorities in Nagorno-Karabakh said 16 of their servicemen had died, with 100 injured.
Armenia said it had shot down two helicopters and three drones, as well as destroying three tanks.
Armenia's government declared martial law and total military mobilisation, shortly after a similar announcement by the authorities inside Nagorno-Karabakh.
Martial law is an emergency measure under which the military takes over the authority and functions of the civilian government.
image copyrightEPAimage captionAzerbaijan released images of what it said were damaged Armenian armoured vehicles
"Get ready to defend our sacred homeland," Armenian Prime Minister Nikol Pashinyan said after accusing Azerbaijan of "pre-planned aggression".
Warning that the region was on the brink of a "large-scale war", and accusing Turkey of "aggressive behaviour", he urged the international community to unite to prevent any further destabilisation.
According to Azerbaijani prosecutors, five members of the same family were killed by Armenian shelling of one village in Azerbaijan.
Azerbaijan's defence ministry confirmed the loss of one helicopter but said the crew had survived, and reported that 12 Armenian air defence systems had been destroyed. It denied other losses reported by Armenia.
President Aliyev said he had ordered a large-scale counter-offensive operation in response to Armenian army attacks.
image copyrightEPAimage captionAzerbaijani President Ilham Aliyev went on TV
"As a result of the counter-offensive operation, a number of Azerbaijani residential areas that were under occupation have been liberated," he said in remarks broadcast on television.
"I am confident that our successful counter-offensive operation will put an end to the occupation, to the injustice, to the 30-year-long occupation."
After initial denials by the Armenian military, Nagorno-Karabakh's unrecognised president, Arayik Harutyunyan, confirmed some positions had been lost to Azerbaijani forces.
Nagorno-Karabakh - key facts
President Erdogan called Armenia "the biggest threat to peace and tranquillity in the region".
The Organisation for Security and Co-operation in Europe (OSCE) has long been trying to mediate a settlement of the conflict, with diplomats from France, Russia and the US - making up the OSCE Minsk Group - trying to build on a 1994 ceasefire.
Just how fragmented is the region?
Azerbaijanis are a predominantly Turkic people with whom Turkey has close ties, although unlike Turks, most Azerbaijanis are Shia, not Sunni, Muslims.
Turkey does not have relations with Armenia, a mainly Orthodox Christian country which has historically looked to Russia for support.
Iran, a mainly Shia state, has a large ethnic Azerbaijani community but maintains good relations with Russia. They and Turkey, a Nato member, back opposing sides in Syria's ongoing civil war.
Since the USSR collapsed in 1991, the ethnic divisions in Armenia and Azerbaijan have become even starker: according to an Armenian report in 2004, just 30 people in Armenia (population 3.1 million) identified as Azerbaijanis, while the 2009 census for Azerbaijan (population 9.7 million) recorded 183 Armenians living in areas other than Nagorno-Karabakh.
A 2015 census for the unrecognised "Republic of Artsakh" - or Nagorno-Karabakh (population 145,053) - records no Azerbaijanis as living there. In Soviet times, they had made up more than a fifth of the region's population.