US House Speaker Nancy Pelosi on Sunday condemned what she described as an "illegal" attack by Azerbaijan on Armenia that sparked the worst fighting since their 2020 war.
Baku and Yerevan have accused each other of starting Tuesday's border clashes, which claimed the lives of more than 200 people.
"We strongly condemn those attacks -- on behalf of Congress -- which threaten (the) prospects of the much-needed peace agreement," Pelosi told journalists in Yerevan.
"Armenia has particular importance to us because of the focus on security following an illegal and deadly attack by Azerbaijan on the Armenian territory."
The attack was an "assault on (the) sovereignty of Armenia", she added.
Hostilities between the Caucasus arch foes ended overnight on Thursday thanks to mediation by the United States, Armenian parliament speaker Alen Simonyan said.
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Earlier attempts by Russia to broker a truce failed.
"We are grateful to the United States for the agreement of the fragile ceasefire reached by their mediation," he told journalists alongside Pelosi.
US Secretary of State Antony Blinken on Sunday also spoke with Azerbaijan's President Ilham Aliyev, according to a State Department readout of their call.
Blinken "urged President Aliyev to adhere to the ceasefire, disengage military forces, and work to resolve all outstanding issues between Armenia and Azerbaijan through peaceful negotiations," said spokesman Ned Price.
Pelosi's visit marks a growing closeness between Washington and Yerevan where frustration is brewing over the lack of support from Armenia's traditional ally Moscow which is distracted by its nearly seven-month war in Ukraine.
Russia -- which has a treaty obligation to defend Armenia in the event of foreign invasion, but which also has close ties with Baku -- did not rush to help Yerevan despite a formal demand for military help.
"We asked for military help and our demand was not accepted. Obviously, we are not happy," Armenia's security council chairman, Artyom Grigoryan, said Friday.
Pelosi, who arrived in Yerevan on Saturday for a three-day visit, is the highest-ranking US official to travel to Armenia since the tiny nation gained independence from the Soviet Union in 1991.
On Sunday morning, a tearful Pelosi laid flowers at Yerevan's hilltop memorial of the 1.5 million Armenians killed in the Ottoman Empire during World War I.
Armenia has long sought international recognition of the bloodletting as genocide -- a claim fiercely rejected by Turkey but supported by many other countries.
Pelosi said she was "proud" to travel to Yerevan after US President Joe Biden formally acknowledged the Armenian genocide last year.
"It is the moral duty of all to never forget: an obligation that has taken on heightened urgency as atrocities are perpetrated around the globe, including by Russia against Ukraine," Pelosi said on Saturday.
Armenia and Azerbaijan have fought two wars -- in the 1990s and in 2020 -- over the contested region of Nagorno-Karabakh, an Armenian-populated enclave of Azerbaijan.
Pelosi said: "In the Congress, in the bipartisan way, we hold (Baku's ally) Turkey responsible -- as well as Azerbaijan -- for the conflict that exists in Nagorno-Karabakh."
Together with France and Russia, the US co-chairs the Minsk Group of mediators, which had led decades-long peace talks between Baku and Yerevan under the aegis of the Organisation for Security and Cooperation in Europe.
The Minsk Group has been largely defunct as Moscow faces growing isolation on the world stage following its February invasion of Ukraine.
The European Union had taken a lead role in mediating the Armenia-Azerbaijan normalisation process.
Analysts have said the hostilities have largely undone Western efforts to bring Baku and Yerevan closer to a peace deal.
The six-week war in 2020 claimed the lives of more than 6,500 troops from both sides and ended with a Russian-brokered ceasefire.
Under the deal, Armenia ceded swathes of territory it had controlled for decades, and Moscow deployed about 2,000 Russian peacekeepers to oversee the fragile truce.
Ethnic Armenian separatists in Nagorno-Karabakh broke away from Azerbaijan when the Soviet Union collapsed in 1991. The ensuing conflict claimed around 30,000 lives.