US President Joe Biden's national security chief will brief top lawmakers Thursday after they warned of a "serious" threat reportedly involving a Russian bid to build a space-based nuclear capability.
White House National Security Advisor Jake Sullivan will hold a closed-doors meeting with congressional leaders, as US media said Russia was developing a weapon that could knock out Western satellites.
Moscow denied the "malicious" and "unfounded" reports, describing them as a White House ploy to try to pass a multi-billion-dollar Ukrainian war aid package that is stalled in Congress.
Mystery and concern has gripped Washington since US House Intelligence Committee chairman Michael Turner abruptly issued a cryptic statement on Wednesday referring to a "serious national security threat."
The Republican asked Biden to declassify all information about the unidentified threat so the United States and its allies could "openly discuss the actions necessary to respond."
American media outlets then reported that it involved Russia advancing plans to deploy a nuclear weapon based in space that could target satellites.
US officials said the threat described did not involve an active capability or one that could target humans, but added that it was significant.
They also pointed to comments by Republican House speaker Mike Johnson saying that there was "no need for public alarm."
But the sudden announcement irked the White House's Sullivan, who signaled frustration that Turner had gone public ahead of a 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.
He refused to confirm he would discuss the security threat, but pointed to the significance by saying it was "highly unusual, in fact," for the national security advisor to "personally" reach out to House members as he had.
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.
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.
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.
"If they make any kind of claim, they should accompany it with evidence," he added," TASS cited Ryabkov as saying Thursday.
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, of 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.
