Yerevan (AFP) - Azerbaijani tanks on Wednesday shelled positions in the breakaway Nagorny Karabakh region for the first time in more than 20 years, the rebel defence ministry said, as Armenia arrested a former army officer accused of spying for its archfoe.

Armenia and Azerbaijan fought a bloody conflict over the disputed Nagorny Karabakh territory in the 1990s before reaching a tenuous ceasefire in 1994 but they have not signed a peace deal and clashes erupt regularly.

The ethnic Armenian enclave in Azerbaijan enjoys the backing of the Armenian authorities.

"For the first time since the ceasefire, Azerbaijan has used tanks on the Karabakh frontline," killing one soldier, the rebel defence ministry said in a statement.

"Some 1,500 shots were fired from tanks and grenade-launchers," the statement added.

Azerbaijan offered a different version of events, saying Armenia had fired mortar rounds at settlements in Azerbaijan.

The defence ministry in Baku warned it would launch retaliatory strikes on "enemy" army positions in Nagorny Karabakh, close to the frontier.

"The regime in Armenia bears responsibility for all of this," Azerbaijan's defence ministry said in a statement.

In Washington, the US State Department condemned the violence and urged all sides to adhere to the ceasefire.

"The recent escalation of violence and the use of heavy weapons are unacceptable," spokesman John Kirby said.

Separately, Armenia said it had arrested a former senior army officer on suspicion of spying for Azerbaijan and selling state secrets.

"A retired major, Garik Marutyan, 38, who previously served as a reconnaissance chief at the defence ministry, cooperated with Azerbaijani special services active in Turkey," Armenia's national security agency said in a statement.

If convicted, he faces a jail term of up to 15 years, it said.

The two ex-Soviet nations regularly exchange fire along their shared border and across Karabakh's volatile frontline.

Raising the spectre of a return to all-out war, both sides reportedly used heavy artillery in tit-for-tat attacks in September.

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Mediators from the Organisation for Security and Cooperation in Europe warned last week that "the status quo has become unsustainable" and called on the two countries to reduce tensions.

US mediator James Warlick tweeted on Friday that Armenian President Serzh Sarkisian and Azerbaijan's Ilham Aliyev "were on track" to hold their first face-to-face encounter in over a year later this month.

Ethnic Armenian separatists backed by Yerevan seized control of Nagorny Karabakh during the 1990s war, in which some 30,000 people died.