The Republican majority leadership in the US Senate Thursday refused the postponement of the scheduled Monday hearing of Judge Brett Kavanaugh, who is facing sexual assault allegations, and set a Friday deadline for Christine Blasey Ford to respond.

In a letter to Democratic members of the Senate Judiciary Committee, its chairman Chuck Grassley also opposed the idea of an FBI investigation, a condition set by Ford for her to appear before the committee for testimony, a move which has been supported by Democratic lawmakers.

In the absence of Ford appearing before the Senate Judiciary Committee, the Republicans Thursday said they will go ahead with the confirmation process and the White House said it will fully support Kavanaugh, the US Supreme Court nominee.

Grassley, in his letter, dismissed the demand of FBI investigation.

"Your letter requests that I demand that the FBI conduct an additional investigation into this matter. This request demonstrates a fundamental misunderstanding of the FBI background investigation process. Before nominating an individual to a judicial or executive office, the White House directs the FBI to conduct a background investigation," he said.

"The FBI compiles information about a prospective nominee and sends it to the White House. The White House then provides FBI background investigation files to the Senate as a courtesy to help us determine whether to confirm a nominee. But the FBI does not make a credibility assessment of any information it receives with respect to a nominee," Grassley argued.

He said he has scheduled the hearing on coming Monday.

Grassley alleged that Senator Dianne Feinstein only informed the FBI of the allegations after they were leaked to the media on the eve of a confirmation vote.

"The proper course of action would have been to investigate Dr Ford's serious allegations as quickly and as thoroughly as possible, as I did as soon as these allegations were made known to me," he said.

The Democrat's request, he said, ignores the fact that Ford has already made her allegations public. The purpose of the background investigation process is to compile information in a confidential manner, Grassley said.

"Confidentiality permits people to speak freely and candidly about the character and qualifications of the nominee. The White House requires the Senate to keep background investigation files private so that people can speak anonymously to investigators if they so desire. Because Dr Ford's allegations are in the public arena, there is no longer a need for a confidential FBI investigation," he said.

Grassley said the Senate Judiciary Committee has offered to hold a hearing with Ford in any format, time and place of her choice.

Senator Lindsay Graham, in a statement, said seeking an FBI investigation into a 36-year-old incident is aimed at delaying the confirmation process.

"Requiring an FBI investigation of a 36-year-old allegation (without specific references to time or location) before Professor Ford will appear before the Judiciary Committee is not about finding the truth, but delaying the process until after the mid-term elections," he said.

"It is imperative the Judiciary committee move forward on the Kavanaugh nomination and a committee vote be taken as soon as possible," Graham said.