WASHINGTON • As the Federal Bureau of Investigation's Russia probe draws closer to him, United States President Donald Trump unleashed an extraordinary assault on the nation's top law enforcement agency.

In a series of early morning tweets on Sunday, Mr Trump said the FBI was a biased institution whose reputation for fairness was "in tatters" and its standing was now the "worst in history".

The attack was one of the harshest in a generation on an independent agency which, just days earlier, had helped secure a guilty plea and pledge of cooperation from Mr Trump's former national security adviser, Mr Michael Flynn.

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 election last year.

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.

Mr 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.

He added that they demonstrated "unwavering integrity and professionalism" on the job.

"Suggesting otherwise is simply false," he said.

Last Friday, Mr Flynn 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, Mr Robert Mueller, is believed to be examining whether Mr Trump obstructed justice by firing then FBI director James Comey, who was overseeing the inquiry. Mr Comey has said Mr Trump asked him to drop the investigation into Mr Flynn.

But on Sunday, Mr Trump condemned Mr Comey as a liar, saying, "I never asked Comey to stop investigating Flynn", and that Mr 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 Mrs Hillary Clinton, his rival during last year's presidential contest.

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

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

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

In several tweets, the President harshly criticised the agent, Mr Peter Strzok, who had previously helped lead last year's investigation into whether Mrs Clinton had mishandled classified information on her private e-mail account.

Mr Strzok is considered one of the bureau's most experienced and trusted counter-intelligence investigators.

Trump, in his 10th tweet on Sunday, said: "Report: 'ANTI-TRUMP FBI AGENT LED CLINTON E-MAIL PROBE.'"

By early evening that day, the tweet had been retweeted more than 24,000 times. "Now it all starts to make sense!"

Mr Trump also appeared to remain especially fixated on Mr Comey, whom he had fired.

In one tweet on Sunday, Mr Trump said: "After years of Comey, with the phony and dishonest Clinton investigation (and more), running the FBI, its reputation is in Tatters - worst in History!"

NYTIMES