WASHINGTON (NYTIMES) - As the FBI's Russia investigation draws closer to him, President Donald Trump on Sunday (Dec 3) unleashed an extraordinary assault on the nation's premier law enforcement agency, calling it a biased institution whose reputation for fairness was "in tatters".

In a series of early-morning tweets, Trump said the FBI's standing was now the "worst in history".

The attack was one of the harshest in a generation on an independent agency that two days earlier had helped secure a guilty plea and a pledge of cooperation from the president's first national security adviser.

Current and former FBI officials, historians and lawmakers rebuked the President over his efforts to undermine the FBI's credibility as it investigates whether his campaign colluded with Russian officials to sway the 2016 election.

A president who has positioned himself as devoted to law and order is now in a public dispute with the country's top law enforcement agents.

Thomas O'Connor, president of the association representing FBI agents, defended their integrity in a statement.

"FBI agents are dedicated to their mission," he said, asserting that they demonstrated "unwavering integrity and professionalism" on the job.

"Suggesting otherwise is simply false," he added.

On Friday, Michael Flynn, the former national security adviser, admitted that he had lied to the FBI about his conversations with the Russian ambassador during the presidential transition.

As part of the bureau's inquiry, the special counsel, Robert Mueller, is believed to be examining whether Trump obstructed justice by firing James Comey, the FBI director, who was overseeing the inquiry. Comey has said Trump asked him to drop the investigation into Flynn.

But on Sunday, the President condemned Comey as a liar, saying that "I never asked Comey to stop investigating Flynn" and that Comey had harmed the bureau and its employees.

He also accused the bureau's agents of spending years pursing a "phony and dishonest" investigation into the e-mail server of his 2016 rival, Hillary Clinton.

Trump's fury at those investigating him stunned even those with fresh memories of his repeated attempts over the past year to disparage intelligence agencies, the State Department and other parts of his government.

Senator Dianne Feinstein, D-Calif., the top Democrat on the Judiciary Committee, said the frenzied nature of the President's tweets suggested that he knew that Mueller was building an obstruction of justice case against him.

"I see it in the hyperfrenetic attitude of the White House, the comments every day, the continual tweets," Feinstein said on NBC's "Meet the Press" on Sunday.

Eric H. Holder Jr., who was former president Barack Obama's first Attorney-General, responded to the President's tweets with one of his own defending the bureau.

"You'll find integrity and honesty at FBI headquarters and not at 1600 Penn Ave right now," Holder wrote.

As he sought to discredit the Russia inquiry, which he has long called a political "witch hunt," Trump on Sunday seized on reports that Mueller had removed a veteran FBI agent because he sent text messages that appeared to express views critical of Trump.

In several tweets, the President harshly criticised the agent, Peter Strzok, who had previously helped lead the 2016 investigation into whether Clinton had mishandled classified information on her private e-mail account. Strzok is considered one of the bureau's most experienced and trusted counterintelligence investigators.

"Report: 'ANTI-TRUMP FBI AGENT LED CLINTON EMAIL PROBE,'" Trump said in his 10th tweet Sunday, which by the early evening had been retweeted more than 24,000 times. "Now it all starts to make sense!"

Most presidents enter the Oval Office with an instinct to defend and promote the integrity and capabilities of the nation's law enforcement agencies. Trump arrived with a different compulsion, fueled by a belief that intelligence and law enforcement officials were stoking questions about the legitimacy of his election victory.

By suggesting - as he has before - that the FBI and other agencies are motivated by politics, Trump again embraced the kind of suspicions that feed conspiracy theories about a "deep state" operating with a secret bias against him.

Still, even though Trump's attacks on government agencies are now a familiar theme, former FBI officials and veteran observers of the agency said they were surprised at the ease with which the President sought to defend himself by attacking the reputations of Comey, Mueller, Strzok and the 35,000 people who work at the FBI.

Robert E. Anderson Jr., a former top spy hunter at the bureau, said the President's comments would have a dispiriting effect on FBI morale, especially among those who are not involved in political investigations.

"You've got men and women working tirelessly in every corner of this world to protect the United States and its people," Anderson said.

"When he says what he says, it's an insult and it's degrading to the men and women who are sacrificing their lives to protect this great nation."

Anderson also came to the defense of Strzok, calling him "one of the most methodical, most meticulous, hardworking counterintelligence experts in the entire United States intelligence community."

Anderson said Strzok "never displayed political bias".

The President retweeted a Twitter post urging Christopher A. Wray, the current FBI director, to "clean house" at the agency.

In a statement on Sunday, Attorney-General Jeff Sessions said he had directed Wray to "review the information available on this and other matters and promptly make any necessary changes".

But Trump appears to remain especially fixated on Comey, who testified before Congress in June that the President had asked him to drop the inquiry into Flynn's activities one day after Trump had fired Flynn. Comey declined to do so, and the President fired Comey several months later.

In one tweet on Sunday, Trump blamed "years of Comey" at the helm of the FBI for what he views as the damage to its reputation.

"After years of Comey, with the phony and dishonest Clinton investigation (and more), running the FBI, its reputation is in Tatters - worst in History!" Trump wrote.

"But fear not, we will bring it back to greatness."

The tweet drew a sharp rebuke from Holder. "The FBI's reputation is not in 'tatters,'" Holder wrote on Twitter.

"It's composed of the same dedicated men and women who have always worked there and who do a great, apolitical job."