Donald Trump has denied being briefed by intelligence officials that the Russians offered bounties to the Taliban to kill British and American soldiers in Afghanistan.
Reports of the Russian bounties, first carried in the New York Times, were confirmed by other major US news outlets over the weekend.
According to the reports Mr Trump, who has sought to forge a close relationship with Vladimir Putin, was briefed about the bounties allegedly paid to Islamic militants in March but failed to act.
The reports were dismissed as “fake news” by the president on Twitter.
“Nobody’s been tougher on Russia than the Trump Administration,” he tweeted.
“With Corrupt Joe Biden & Obama, Russia had a field day, taking over important parts of Ukraine – Where’s Hunter? Probably just another phony Times hit job, just like their failed Russia Hoax. Who is their “source”?”
Lagging in the polls and under fire for his response to the coronavirus crisis and recent unrest in the US, Mr Trump has found himself vulnerable to accusations of failing to protect US national security.
The Foreign Office, meanwhile, declined to comment on security matters.
John Bolton, whose newly-published memoirs paint an unflattering picture of the Trump administration, stepped up his attack on his former boss as the row over what the president knew intensified.
Appearing on CNN’s State of the Union Mr Bolton, who was sacked by the US president last September, was unable to confirm the briefings took place and suggested the reports should be treated with caution.
However, Mr Trump’s response was purely political “because it looks bad if Russians are paying to kill Americans and we’re not doing anything about it”, Mr Bolton added.
“The presidential reaction is to say, it’s not my responsibility nobody told me about it. And therefore to duck, any complaints that he hasn’t acted.”
Mr Bolton continued: “The fact that the President feels compelled to tweet about the news story here shows that what his fundamental focus is is not the security of our forces, but whether he looks like he wasn’t paying attention.”
At least 20 US servicemen have been killed in Afghanistan, but it remains unclear whether any were victims of the bounty operation which was reportedly conducted by Unit 29155 of Russia’s GRU military intelligence agency.
The same unit has been accused of conducting the nerve agent attack on former Russian spy Sergei Skripal in Salisbury in 2018.
Both the Russian foreign ministry and the Taliban have strongly denied the allegations.
Joe Biden, his Democratic opponent in November’s presidential election, said he was “outraged” by the report.
“This is beyond the pale. It’s a betrayal of the most sacred duty in the nation: to protect our troops when we send them into harm’s way.”
Nancy Pelosi, the Speaker of the House, described Mr Trump’s response as “totally outrageous”.
She told ABC News: “You would think that the minute he heard of this the president would want to know more instead of denying anything.”