WASHINGTON - US President Donald Trump has ordered a new FBI investigation into sexual assault allegations against his Supreme Court pick, as the Senate delayed a vote on Brett Kavanaugh's nomination to make way for the probe.

After a dramatic day-long hearing at which a university professor accused Kavanaugh of pinning her down and assaulting her at a party in the 1980s, the Senate Judiciary Committee earlier Friday approved his nomination in a preliminary vote along party lines.

But committee chair Chuck Grassley, a Republican, then bowed to intense pressure from inside and outside Congress and announced he was asking Trump to order the new background review that "must be completed no later than one week from today."

The Kavanaugh nomination has turned into a political firestorm ahead of November congressional elections - in which Republicans will battle to keep control of Congress - and is threatening to derail Trump's push to get a conservative-minded majority on the top court ahead of the vote.

Trump, who repeatedly blasted Democratic opposition to Kavanaugh as a political "con-job," agreed to the investigation. "I've ordered the FBI to conduct a supplemental investigation to update Judge Kavanaugh's file.

As the Senate has requested, this update must be limited in scope and completed in less than one week," the president said in a statement.

He later tweeted support for Kavanaugh, saying "Just started, tonight, our 7th FBI investigation of Judge Brett Kavanaugh. He will someday be recognized as a truly great Justice of The United States Supreme Court!"

Trump's order and the decision to put off a final vote came after a key Republican senator, Jeff Flake, threatened to break ranks with his party because of the clouds hanging over Kavanaugh.

At least three women have now accused the 53-year-old conservative judge of sexual misconduct while drunk, as a high school and later a university student.

Democrats have repeatedly demanded those charges be assessed by the Federal Bureau of Investigation to determine if he is suitable for the lifetime high court appointment.

Flake's 11th-hour move played out on national television amid heated partisan debate and raucous protests in the halls of the Capitol, where several women cornered the Arizona senator in an elevator demanding he oppose Kavanaugh.

"This country's being ripped apart," said Flake. "And we have to ensure that we do due diligence here."

"There's lingering doubt out there among a lot of people that we haven't taken every measure that we should to make sure that these allegations are dealt with, and so that's what this effort is about," he said.