Azerbaijan's defence ministry says that five of its soldiers have been killed in clashes with Armenian troops along the border separating the two countries, deepening tensions between the two former Soviet nations.
The ministry said in a statement on Tuesday that exchanges of gunfire had been reported over the last two days at numerous points along Azerbaijan's western border.
Armenia said earlier that three of its soldiers died in the clashes.
Azerbaijan and Armenia have for two decades been at odds over the disputed Nagorno-Karabakh territory, which lies within Azerbaijan, but was taken over by Armenia during a six-year war that killed about 30,000 people and displaced one million.
The incidents come as Hillary Clinton, the US secretary of state, embarked on a tour of the South Caucasus in the hope of mediating progress in territorial disputes in the region.
Azerbaijan's defence ministry said one clash took place near the village of Ashagy Askipara early on Tuesday morning, after their soldiers were attacked by Armenian commandos.
Four Azeri troops were killed in the fighting, officials said. Another soldier died in a separate incident, the ministry said.
Armenia on Monday said three of its soldiers were killed and another six were wounded in villages nearby.
Clinton decried the "senseless deaths of young soldiers and innocent civilians" as part of the Nagorno-Karabakh conflict - just hours after Monday's border clash.
"I am very concerned about the danger of escalation of tensions and the senseless deaths of young soldiers and innocent civilians," Clinton told reporters after a dinner with Armenia's president and foreign minister.
"The use of force will not resolve the Nagorno-Karabakh conflict," she said, urging the sides to refrain from violence.
Despite years of negotiations since a 1994 ceasefire, the two sides have not yet signed a final peace deal and there are still frequent exchanges of gunfire along the frontline.
Azerbaijan has threatened to use force to win back Karabakh if peace talks fail to yield satisfactory results, but Armenia has warned of large-scale retaliation against any military action.
Warning that Azeri-Armenian tensions could escalate into a broader conflict with terrible consequences, Clinton said the US would continue to press with France, Russia and others on mediation efforts.
Violations of the ceasefire have been frequent, and diplomatic efforts to solve the conflict have failed. The US hopes that at the least Armenia and Azerbaijan can agree to a set of basic principles that might lead to peace. These include the return of territories and uprooted people to their homes, and an eventual vote on the area's future.
On Wednesday Clinton is due to visit Baku, where officials said that finding a resolution to the Karabakh conflict would be the main topic of discussion.