Joe Biden has attacked Donald Trump over a report that he said potentially contains a "truly shocking revelation" about the US president and his failure to protect American troops in Afghanistan and stand up to Russia.
The New York Times reported on Friday American intelligence officials concluded months ago that a Russian military intelligence unit secretly offered bounties to Taliban-linked militants for killing US troops in Afghanistan.
The report said the Russians offered rewards for successful attacks last year, at a time when Washington and the Taliban were holding talks to end the long-running war.
During a virtual town hall, Mr Biden said: "The truly shocking revelation that if the Times report is true, and I emphasise that again, is that President Trump, the commander-in-chief of American troops serving in a dangerous theatre of war, has known about this for months, according to the Times, and done worse than nothing."
The White House said neither Mr Trump nor Vice President Mike Pence were briefed on such intelligence.
Press secretary Kayleigh McEnany said in a statement: "This does not speak to the merit of the alleged intelligence but to the inaccuracy of the New York Times story erroneously suggesting that President Trump was briefed on this matter."
Russia’s foreign ministry has called the report "nonsense".
The New York Times quoted a Taliban spokesperson denying that its militants have a deal with the Russian intelligence agency over bounties.
The newspaper, citing unnamed officials familiar with the intelligence, said the findings were presented to Mr Trump and discussed by his National Security Council in late March.
Officials developed potential responses, starting with a diplomatic complaint to Russia, but the White House has yet to authorise any step, the newspaper reported.
Mr Biden slammed his likely opponent in November over his reported failure to act.
"Not only has he failed to sanction and impose any kind of consequences on Russia for this egregious violation of international law, Donald Trump has continued his embarrassing campaign of deference and debasing himself before Vladimir Putin," the former vice president said.
Mr Biden called it a "betrayal of the most sacred duty we bear as a nation - to protect and equip our troops when we send them into harm’s way".