At least 30 soldiers and a boy have been reported killing in heavy fighting between Armenian and Azerbaijani forces in the disputed Nagorny-Karabakh region.
The fighting is considered the worst outbreak since a full-scale war over the region ended in 1994. The mountainous area, officially part of Azerbaijan, has been under the control of the Armenian separatists since the ceasefire.
Russian President Vladimir Putin urged both sides to cease firing and "show restraint", Kremlin spokesman Dmitry Peskov told Russian news agencies.
In a statement, Azerbaijan's Defence Ministry said 12 of its soldiers "became shahids" - or martyrs - and said one of its helicopters was shot down.
The statement also claimed more than 100 Armenian forces were killed or wounded and six tanks and 15 artillery positions were destroyed. The claim has not been verified.
Armenian President Serzh Sargysyan told his national security council 18 Armenian soldiers were killed and 35 wounded.
Armenia had earlier claimed to have inflicted heavy damage on Azerbaijani forces, but did not immediately give figures.
Each sides blame the other for the escalation in conflict.
The Armenian Defence Ministry claimed Azerbaijan used aircraft, tanks and artillery to capture territory in the Nagorno-Karabkh area, while the Azerbaijani Defence Ministry said the fighting began when Armenian forces fired mortars and artillery shells across the front line.
Clashes have erupted regularly between the two countries since the 1994 ceasefire. The Nagorno-Karabakh war claimed more than 20,000 lives.
Additional reporting by agencies
