Five more soldiers were killed in skirmishes between Azeri government forces and ethnic Armenian separatists controlling the breakaway Nagorno-Karabakh enclave of Azerbaijan, officials from both sides said on Saturday.
The casualties bring the death toll to at least 15 in a flare-up of violence over the last few days.
Azerbaijan's Nagorno-Karabakh region and some adjacent territory have been under the control of Armenian soldiers and ethnic Armenian local troops since the end of a six-year separatist war in 1994. Both sides report frequent shootings and attempted incursions along the cease-fire line, but the latest outbreak of violence is the worst in many years.
The clashes have underlined the risk of broader conflict in the South Caucasus, where vital oil and natural gas flow from the Caspian region to Europe. Russia, an ally of Armenia, issued a statement on Saturday warning against further escalation, raising fears among some that it may get involved in the conflict.
"We see the events of recent days as a serious violation of agreements on a cease-fire and declared intentions to achieve a regulation [of the conflict] through political means," the Russian Foreign Ministry said in a statement on Saturday.
"We take the position that any further escalation is unacceptable," it said.
Russia's annexation of Crimea, however, has contributed to the tensions. Armenia, which depends on Russia for economic and military support, has welcomed the takeover of Crimea and some Armenians have suggested it could be a model for Nagorno-Karabakh. This has rattled Azerbaijan, which like Ukraine has aligned itself with the West.
Energy-producing Azerbaijan, host to oil majors including BP, Chevron and ExxonMobil, frequently threatens to take Nagorno-Karabakh back by force and is spending heavily on its armed forces. Armenia, an ex-Soviet republic like Azerbaijan, has warned it will intervene if Nagorno-Karabakh is overrun.
Azerbaijan's Defense Ministry said on Saturday four of its soldiers had been killed in overnight skirmishes along the line that demarcates Nagorno-Karabakh from Azerbaijan.
"Armenian sabotage groups attacked positions of the Azeri army," the Azeri ministry said in a statement.
The breakaway enclave also said one of its combatants were killed in the fighting. On Friday, Azerbaijan reported about eight soldiers killed and the separatists said two of their soldiers had been killed.
Armenia blamed Azerbaijan for the latest bloodshed. "Such behavior by the Azeri armed forces contradicts the logic of negotiating a resolution of the conflict and may lead to large-scale military actions," the Armenian Defense Ministry said in a statement.
Nagorno-Karabakh runs its own affairs with heavy military and financial backing from Armenia since the war that killed about 30,000 people two decades ago. Armenian-backed forces also seized seven Azeri districts surrounding Nagorno-Karabakh. Efforts to reach a permanent settlement of the Nagorno-Karabakh conflict have failed despite mediation led by France, Russia and the United States.