FBI Director Christopher Wray has announced his decision to resign at the end of US President Joe Biden's term in January, paving the way for President-elect Donald Trump's nominee Indian-American Kash Patel to take charge of the country's top investigating agency.

The move has been welcomed by Trump.

"After weeks of careful thought, I have decided the right thing for the bureau is for me to serve until the end of the current administration in January and then step down," Wray told his FBI colleagues during a town hall here on Wednesday.

On November 30, Trump nominated his loyalist and staunch supporter Patel as the next FBI director, thus cutting short Wray's 10-year term by three years. Wray was appointed by Trump during his first term in 2017.

The FBI director told his colleagues that his goal is to keep the focus on the mission — "the indispensable work they are doing on behalf of the American people every day".

"And in my view, this is the best way to avoid dragging the bureau deeper into the fray, while reinforcing the values and principles that are so important to how we do our work," he said.

"It should go without saying, but I will say it anyway: This is not easy for me. I love this place. I love our mission. And I love our people. But my focus is, and always has been, on us and doing what is right for the FBI. And it was important to me that you all got to hear about my decision directly from me before it gets announced publicly," Wray said.

Welcoming the decision, Trump said, "The resignation of Christopher Wray is a great day for America as it will end the weaponisation of what has become known as the United States Department of Injustice."

"I just do not know what happened to him. We will now restore the rule of law for all Americans. Under the leadership of Christopher Wray, the FBI illegally raided my home, without cause, worked diligently on illegally impeaching and indicting me, and has done everything else to interfere with the success and future of America," he said.

"They have used their vast powers to threaten and destroy many innocent Americans, some of which will never be able to recover from what has been done to them," the president-elect added.

Trump said Patel was the most qualified nominee to lead the FBI in the agency's history.

"As everyone knows, I have great respect for the rank and file of the FBI, and they have great respect for me. They want to see these changes every bit as much as I do but, more importantly, the American people are demanding a strong, but fair, system of justice. We want our FBI back, and that will now happen. I look forward to Kash Patel's confirmation so that the process of Making the FBI Great Again can begin," he said.

Attorney General Merrick B Garland said Wray has served the country honourably and with integrity for decades, including for seven years as the director of the FBI under presidents of both parties.

"Under Director Wray's principled leadership, the FBI has worked to fulfil the Justice Department's mission to keep our country safe, protect civil rights, and uphold the rule of law," he said.

Senate Majority Whip Dick Durbin, who is also chair of the Senate Judiciary Committee, said the FBI will soon embark on a perilous new era with serious questions about its future.

"I want to thank Director Wray for his service to our nation, and all the men and women of the FBI for their continued efforts to protect our security and liberty," he said.

"Wray's departure is an opportunity for a new era of transparency and accountability at the FBI. Future FBI directors ought to learn a lesson from Wray's mistakes. Stonewalling Congress, breaking promises, applying double standards and turning your back on whistleblowers is no longer going to cut it," Senator Chuck Grassley said.