"Such a demand is unacceptable and cannot be part of a negotiation," Romania's foreign ministry said in a press release.

Bulgarian President Rumen Radev, who is the country's commander in chief of the armed forces and is considered pro-Russian, said in a statement: "Russia's insistence on the withdrawal of NATO forces from Bulgaria is unacceptable and pointless."

On Friday, ahead of crucial Russia-US talks in Geneva, Moscow called for the "withdrawal of foreign forces, hardware and arms" from countries that were not NATO members before 1997.

"These include Bulgaria and Romania," the Russian foreign ministry said in a statement, singling out the two former Warsaw Pact allies who joined NATO in 2004.

Romania said NATO's presence in allied countries was "a purely defensive reaction to Russia's increasingly aggressive behaviour in the eastern vicinity... currently intensifying despite NATO's attempts to engage in constructive dialogue."

Romania, which already hosts around 1,000 US troops on its territory, said on Thursday that it is ready to welcome additional ones, as well as French troops, following an announcement from France's President Emmanuel Macron.

Under several bilateral agreements with the United States, Bulgaria routinely hosts up to 2,500 US troops for exercises at a training area in the east of the country, with troop deployments allowed to reach up to 5,000 during rotation.

The country has also hosted NATO training sessions.

Bulgaria's Radev noted: "There are no permanently based allied contingents or combat equipment in the country."

But "the sovereign decision for such a permanent presence or temporary deployment belongs to Bulgaria, in accordance with our allied commitments," he added.

"Our country does not accept ultimatum demands from anyone."

Bulgarian Prime Minister Kiril Petkov also told parliament earlier on Friday: "Bulgaria is a sovereign country and we have long made our choice by becoming a member of NATO."

"As such, we decide alone how to organise the defence of our country in coordination with our partners," he said.

"We call on the Russian Federation to remain engaged in constructive dialogue to de-escalate tension."