WASHINGTON - Days before he was fired by Donald Trump, FBI Director James Comey requested more resources to pursue his investigation into Russia's election meddling and the possible involvement of Trump associates, US officials said on Wednesday.

It was unclear whether word of the Comey request, put to Deputy Attorney General Rod Rosenstein, ever made its way to Trump. But the revelation intensified the pressure on the White House from both political parties to explain the motives behind Comey's ouster.

Trump is the first president since Richard Nixon to fire a law enforcement official overseeing an investigation with ties to the White House. Democrats quickly accused Trump of using Comey's handling of the Hillary Clinton email investigation as a pretext and called for a special prosecutor into the Russia probe. Republican leaders brushed off the idea as unnecessary.

Defending the firing, White House officials said Trump's confidence in Comey had been eroding for months. They suggested Trump was persuaded to take the step by Justice Department officials and a scathing memo, written by Rosenstein, criticizing the director's role in the Clinton investigation.

"Frankly, he'd been considering letting Director Comey go since the day he was elected," White House spokeswoman Sarah Huckabee Sanders said, a sharply different explanation from the day before, when officials put the emphasis on new Justice complaints about Comey.

Trump's daring decision to oust Comey sparked comparisons to Nixon, who fired the special prosecutor running the Watergate investigation that ultimately led to his downfall. And Trump's action left the fate of the Russia probe deeply uncertain.

The investigation has shadowed Trump from the outset of his presidency, though he's denied any ties to Russia or knowledge of campaign coordination with Moscow.

Trump, in a letter to Comey dated on Tuesday, contended that the director had told him "three times" that he was not personally under investigation. The White House refused Wednesday to provide any evidence or greater detail. Former FBI agents said such a statement by the director would be all but unthinkable.

Outraged Democrats called for an independent investigation into the Trump campaign's possible ties to Russia's election interference, and were backed by a handful of Republican senators.

But Senate Majority Leader Mitch McConnell, with the support of the White House, brushed aside those calls, saying a new investigation would only "impede the current work being done".

The Associated Press