The announcement came a day after a wave of speculation sparked by US lawmakers making a cryptic warning of an unspecified but serious national security threat to the country.

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 is developing," 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.

Kirby confirmed that the Russian weapon was "space-based" but would not comment on reports in US media that it was either nuclear-capable or nuclear powered.

But he said it would breach the 1967 Outer Space Treaty, which both Russia and the United States are parties to -- and which bans the deployment of nuclear weapons in space.

The weapon could pose a lethal risk to astronauts in low orbit, along with potentially disrupting vital military and civilian satellites, he added.

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

US President Joe Biden had been kept informed and had ordered officials to reach out to Moscow over the weapon, although it had not responded yet.

Concern had gripped Washington since Republican 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' -

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

The early disclosure by lawmakers however irked the White House, with Kirby saying they were working out whether US intelligence sources and methods had been compromised.

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 Mike 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 claims about the technology, calling them 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.

Kirby said the Kremlin's comments were "bollocks."

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 vote.

"Mr Putin should just stay out of our elections," added Kirby.