Armenian forces killed an Azerbaijani soldier on Saturday in a fresh border clash between foes locked in longstanding conflict over the Nagorny Karabakh region, officials in Baku said.

"Lieutenant Bakhyshly Allahverdy, 23, was killed as Armenian forces violated a ceasefire and shelled Azerbaijani army positions on September 27," Azerbaijan's defence ministry said in a statement.

Last month saw an unprecedented spiral of violence with more than 20 troops killed from both sides -- the deadliest clashes since a ceasefire was agreed in 1994.

Nagorny Karabakh broke away from Azerbaijan with the help of Armenia in a war that claimed some 30,000 lives between 1991 and 1994.

Despite two decades of negotiations, the two sides have yet to sign a peace deal.

Azerbaijan has threatened to take back the region by force if negotiations do not yield results, while Armenia has vowed to retaliate against any military action.