US Secretary of State Hillary Clinton has voiced concern over border clashes between Armenia and Azerbaijan - hours after three Armenian soldiers died in the latest skirmishes.
Speaking in Armenia, she warned that the clashes "could escalate into a much broader conflict" in the Caucasus.
The two former Soviet republics fought a bloody war over the disputed region of Nagorno-Karabakh in the 1990s.
A formal ceasefire is now looking increasingly fragile.
"I am very concerned by these incidents and have called on all parties, all actors, to refrain from the use or threat of force," Mrs Clinton said after her talks in the Armenian capital, Yerevan, on Monday.
"There is a danger that it could escalate into a much broader conflict that would be very tragic for everyone concerned," she warned.
The BBC's Damien McGuinness, in neighbouring Georgia, says it is hoped Mrs Clinton will ease tensions between the two countries.
The region is dependent on US aid, so Washington has much influence there, he says.
The conflict between Armenia and Azerbaijan erupted as the Soviet Union collapsed and at least 30,000 people were killed by the time the truce was declared in 1994.
Since then, a simmering stalemate has prevailed and both sides have reported sporadic breaches of the truce.
The three Armenian soldiers were killed in clashes on Monday, Yerevan said.
It remains unclear if there were any casualties on the Azeri side.
Both sides have been building up arms in recent months and refusing to compromise.
It is hoped Mrs Clinton will at least call on both governments to tone down the rhetoric and hold peace talks.
Human rights groups also want Mrs Clinton to address alleged abuses in the region.
Azerbaijan's harsh treatment of government critics has been in the spotlight since it hosted the Eurovision Song Contest in May.
Authorities have dismissed criticism in the international press as Armenian propaganda.
However, condemnation from the US secretary of state will carry more weight, our correspondent adds.
A social media activist was released early from an Azeri jail on Monday, for good behaviour.
Bakhtiyar Hajiyev had been serving a two-year prison term on charges of avoiding military service.
He told the AFP news agency that his arrest had been "politically motivated".
He had used Facebook to support anti-government protests.
Last month a group of US senators demanded Mr Hajiyev's release.
Mrs Clinton travels to Azerbaijan on Wednesday, after visiting Georgia on Tuesday.