The Caucasus neighbours have been locked in a decades-long conflict over the Nagorno-Karabakh region, which Azerbaijan reclaimed after a lightning offensive against Armenian separatists in September.

Both countries have said a peace agreement could be signed by the end of the year, but peace talks – mediated separately by the European Union, the US, and Russia – have seen little progress.

Azerbaijan’s president Ilham Aliyev said today that the US had “nullified” strategic ties between Washington and Baku, adding, “We don’t know why”.

“If the United States says our relationship will no longer be the same, that means Washington is pulling out from mediating the Azerbaijani-Armenian peace process,” he added, saying a change in posture from Washington would mean Baku could “restore” US-led talks.

The comments came as assistant secretary of state James O’Brien was expected in Baku for talks aimed at “strengthening ties” and “supporting the peace between Azerbaijan and Armenia”.

Azerbaijan has refused to participate in talks with Armenia that were planned in the US on Nov 20, over what it said was Washington’s “biased” position.

O’Brien last month said Azerbaijan’s operation to recapture Karabakh, displacing tens of thousands of people, had led Washington to cancel high-level contacts.

In October, Aliyev refused to attend a round of negotiations with Armenia’s prime minister Nikol Pashinyan in Spain, citing France’s “biased position.”

France’s president Emmanuel Macron and Germany’s chancellor Olaf Scholz had been scheduled to join EU chief Charles Michel as mediators at those talks.

So far, there has been no visible progress in EU efforts to organise a fresh round of negotiations.