There were no indications early Thursday whether the two sides had resolved the newest source of tensions, Azerbaijan's new checkpoint on the Lachin Corridor, the only land link between Armenia and Nagorno-Karabakh, the Armenian-majority enclave inside Azerbaijan.

US Secretary of State Antony Blinken was to host later Thursday the closing session of the four-day talks, held in a secure State Department facility just outside Washington.

Late Wednesday US National Security Advisor Jake Sullivan met with Armenian Foreign Minister Ararat Mirzoyan and Azerbaijani Foreign Minister Jeyhun Bayramov at the White House.

"We welcome the progress Armenia & Azerbaijan have made in talks & encourage continued dialogue," he wrote in a statement on Twitter.

"A sustainable & just agreement will be key to unlocking opportunities for both countries & the region," he said.

The new Lachin corridor checkpoint injected new worries over the possible eruption of fighting.

The two sides have gone to war twice, in 1990 and 2020, leaving tens of thousands dead, and clashes regularly erupt over Nagorno-Karabakh.

Ahead of the talks a US official, speaking on grounds of anonymity, said they aim more at "an agreement on normalization of relations" rather than a peace treaty.

"Our goal is to make sure the ministers can sit down and talk to each other," the official said.

But on Tuesday Russia said there was "no alternative" to a deal it signed with the two warring countries in 2020.

"For the moment, there is no other legal basis that would help a resolution. There is no alternative to these trilateral documents," Kremlin spokesman Dmitry Peskov told reporters.

Initiatives to lower tensions in the region "are possible above all on the basis of the trilateral documents signed with Russia," he said.