YEREVAN, Armenia (AP) — Heavy fighting has broken out between Armenian and Azerbaijani forces along the front lines of the separatist Nagorno-Karabakh region, reportedly killing at least one child in what one official called the worst clashes since 1994.

Officials from each former Soviet republic blamed the other Saturday for the fighting that began overnight. Both Armenia and Azerbaijan said they had inflicted heavy losses on the other.

David Babayan, a spokesman for Nagorno-Karabakh's separatist president, said a boy of about 12 was killed and two other children wounded in a Grad missile barrage by Azerbaijani forces. He characterized the fighting as the worst since 1994, when a war over the territory ended.

Russian President Vladimir Putin, meanwhile, urged all sides to cease firing and "show restraint," Kremlin spokesman Dmitry Peskov was quoted as saying by Russian news agencies.

Nagorno-Karabakh, a region in Azerbaijan, has been under the control of local ethnic Armenian forces and the Armenian military since 1994. Years of negotiations under the Organization for Security and Cooperation in Europe have brought little progress in resolving the territorial dispute.

Armenian forces also occupy several areas outside Nagorno-Karabakh. The sides are separated by a demilitarized buffer zone, but both claim frequent violations by the other.

The Armenian Defense Ministry said Azerbaijan used aircraft, tanks and artillery to try to make inroads into Nagorno-Karabkh and that "Azerbaijani authorities bear all responsibility for the unprecedentedly supercharged situation."

The Azerbaijani Defense Ministry said the fighting began when Armenian forces fired mortars and large-caliber artillery shells across the front line. Ministry spokesman Vagif Dargyakhly told The Associated Press that more than 120 shots were fired, some of which hit civilian residential areas.

Dargyakhly also denied a claim by Armenian Defense Ministry spokesman Artsrun Hovhannisyan that an Azerbaijani helicopter was downed in the fighting.