The United States is now reaching out to Moscow over the weapon, whose existence was confirmed after lawmakers warned of an unspecified but serious threat to national security, it added.

Moscow denied the "malicious" and "unfounded" claims, describing them as a White House ploy to try to pass a multi-billion-dollar Ukrainian war aid package stalled in Congress.

"I can confirm that it is related to an anti-satellite capability that Russia has developed," National Security Council spokesman John Kirby told reporters in a briefing at the White House.

"This is not an active capability that's been deployed. And though Russia's pursuit of this particular capability is troubling, there is no immediate threat to anyone's safety," he said.

"We are not talking about a weapon that can be used to attack human beings or cause physical destruction here on Earth."

The United States was nevertheless "closely monitoring this Russian activity and we will continue to take it very seriously".

US President Joe Biden had ordered officials to launch "direct diplomatic engagement" with Russia over the weapon, but there had been no contact yet with Moscow, Kirby said.

Kirby would not confirm reports that the Russian anti-satellite capability was a space-based nuclear weapon.

White House National Security Advisor Jake Sullivan was holding a closed-doors meeting with congressional leaders on Thursday about the developments.

Mystery and concern has gripped Washington since House Intelligence Committee chairman Mike Turner issued a public statement on Wednesday referring to a "serious national security threat" and calling on Biden to "declassify all information relating to this threat."

- 'Tricks' -

The sudden announcement irked Sullivan, who signaled frustration that Turner had gone public ahead of the briefing already planned for Thursday.

Sullivan said he would be meeting with the four House members in the "Gang of Eight" group of party leaders and top intelligence committee members, adding that it was "highly unusual" for him to seek such a meeting.

Democrat Biden and the Republican-led House are at an impasse over a White House request for $60 billion in military aid to help Ukraine's defense against the Russian invasion entering its third year.

Republican House of Representatives speaker Johnson -- who has repeatedly warned he will not address allies' security until America's immigration system is shored up -- is refusing to bring the Senate-passed bill to the floor for a vote.

Kirby said Ukraine's embattled frontline town of Avdiivka was "at risk of falling into Russian control,", partly due to a lack of ammunition.

Moscow dismissed the threat warning as a US attempt to denigrate Russia and push through the Ukraine funding.

Kremlin Spokesman Dmitry Peskov said the White House was "trying to get Congress to vote on the appropriations bill any way it can", the state-run TASS news agency reported.

"It's obvious. Let's see what tricks, so to speak, the White House is going to pull," he was quoted as saying.

Deputy Foreign Minister Sergei Ryabkov, who is involved in Russia's nuclear policy, said the United States was "fantasizing" and should provide evidence of its claims.

The West has accused Russia of reckless nuclear rhetoric after President Vladimir Putin said he was prepared to use a nuclear weapon if he felt an existential threat.

The Outer Space Treaty, which both Russia and the United States are parties to, bans the deployment of nuclear weapons in space.

The row meanwhile came as Putin said he preferred the "predictable" Biden over Donald Trump in November's US presidential election.