Armenia accused Azerbaijani troops of crossing the southern border and trying to stake claim to territory, in a new escalation of tensions between the arch foes.
Last year Armenia and Azerbaijan went to war over the breakaway region of Nagorno-Karabakh. The six-week conflict claimed more than 6,000 lives and ended after Armenia ceded territory it had controlled for decades.
In the early hours of Thursday, Armenia's acting prime minister Nikol Pashinyan accused Azerbaijan of new transgressions as he convened an emergency meeting of his security council.
Mr Pashinyan said Azerbaijan's troops had advanced more than three kilometres into southern Armenia and were trying to "lay siege" to Lake Sev Lich, which is shared by the two countries.
"It is an encroachment on the sovereign territory of the Republic of Armenia," Mr Pashinyan said. "This is an act of subversive infiltration."
He said Armenian soldiers had responded with "appropriate tactical manoeuvres" but stressed that the latest tensions should be settled through negotiations.
Azerbaijan's foreign ministry termed Mr Pashinyan's claims provocative, saying the country's "border troops are taking positions that belong to Azerbaijan, in the Lachin and Kalbajar districts".
The reaction of Armenia, "which makes provocative statements – is astonishing", the ministry said.
"Azerbaijan is committed to defusing the tensions in the region and urges [Armenia] to take steps in that direction."
Armenia, which had controlled Lachin and Kalbajar since the 1990s, handed the districts to Baku last year under the ceasefire agreement.
The US, one of the three nations in the so-called Minsk Group that leads diplomacy on Nagorno-Karabakh, said it was "closely following" the rising tensions.
"We understand communication between the parties is ongoing and urge restraint in de-escalating the situation peacefully," State Department spokesman Ned Price said on Twitter.
French President Emmanuel Macron expressed France's solidarity with Armenia, saying: "Azerbaijani armed forces invaded Armenian territory."
In an Armenian-language post on Facebook, Mr Macron said: "They must withdraw immediately."
Mr Pashinyan has been under huge pressure over his handling of the conflict. He resigned in April but stayed on in a caretaker capacity, setting the stage for June 20 parliamentary elections aimed at defusing the political crisis at home.
Moscow brokered a peace deal between Armenia and Azerbaijan but tensions persist. Russian President Vladimir Putin stressed the need for the ceasefire agreement between Armenia and Azerbaijan to be strictly observed during a phone call with Mr Pashinyan, the Kremlin said on Friday.
Last month, Yerevan and Baku traded accusations of opening fire in Karabakh and along their shared border.
Ethnic Armenian separatists declared independence for Azerbaijan's Nagorno-Karabakh region and seized control of the mountainous enclave in a brutal war in the 1990s that left tens of thousands dead and forced hundreds of thousands from their homes.