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 initiating the border clashes on Tuesday, 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 a news conference 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."

Pelosi said the attack was an "assualt on (the) sovereignty of Armenia".

Hostilities between the Caucasus arch foes ended overnight on Thursday thanks to mediation by the United States, Armenian parliament speaker Alen Simonyan said. 

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 a news conference alongside Pelosi.

Simonyan thanked the US for "the targeted assessment to (sic) the war actions of Azerbaijan".

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.

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.

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.

Analysts have said the recent fighting has largely undone Western efforts to bring Baku and Yerevan closer to a peace deal.

The six-weeks 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.

(AFP)