The US Senate has confirmed Kash Patel as FBI director, putting the former prosecutor in charge of a law enforcement agency that he has fiercely attacked.

The upper chamber of Congress narrowly approved President Donald Trump's nominee by 51-49. Two Republicans voted no.

Democrats had warned he would seek retribution against Trump's enemies, although the nominee has denied any plans to pursue political vendettas.

In his confirmation hearings, Patel, 44, also denied keeping a list of "deep state" enemies, while playing down controversial past remarks he had made such as referring to law enforcement officials who investigated Trump as "criminal gangsters".

Patel was voted against by all Democratic senators and two Republicans who have often dissented, Susan Collins of Maine and Lisa Murkowski of Alaska.

He was the 18th cabinet official approved by lawmakers since Trump was inaugurated a month ago.

"I am honoured to be confirmed as the ninth Director of the Federal Bureau of Investigation," Patel said on X, shortly after he was confirmed by the Senate.

"My mission as Director is clear: let good cops be cops - and rebuild trust in the FBI."

Patel takes the helm of an FBI in flux after the justice department forced out some of its top officials and demanded the names of agents who investigated the 2021 riot by Trump supporters at the US Capitol.

News of Patel's confirmation was greeted by an ovation on the other side of Washington DC where Republicans were gathering for the annual Conservative Political Action Conference (CPAC).

Florida Senator Rick Scott stood on CPAC's stage and asked the crowd: "What do you think about Kash Patel?" Loud applause was the response.

To most Republicans, Patel is a disruptor who will lead an overdue shake of the nation's top law enforcement agency, which they and Trump claim has been weaponised against conservatives.

But Democrats say Patel is a far-right conspiracy theorist with little experience in law enforcement and someone who would put fealty to Trump above his oath to lead a department that is meant to operate independently.

"[Patel's] poor judgment and lack of experience make him wholly unqualified to keep Americans safe as our FBI Director. I voted no," Senator Martin Heinrich said on X.

Patel replaces Christopher Wray who Trump picked in 2017. Wray resigned before inauguration day last month, after the incoming Republican president indicated he would fire him.