Washington (AFP) -

FBI Director Christopher Wray defended his agency Thursday against  President Donald Trump's criticisms, amid a mounting Republican push aimed at discrediting the probe into alleged collusion between the US leader's campaign and Russia in last year's election.

Trump laid into the Federal Bureau of Investigation last weekend, accusing its agents of political bias and Wray's predecessor James Comey, whom Trump fired in May, of lying.

"Its reputation is in Tatters -- worst in History," Trump wrote on Twitter of the FBI, at the end of series of posts about a senior agent who allegedly supported Trump's Democratic rival Hillary Clinton.

Wray responded Thursday in testimony before the House Judiciary Committee: "My experience has been that our reputation is quite good."

"The agents, analysts and staff of the FBI are big boys and girls. We understand that we will take criticism from all corners. We're accustomed to that," he said.

But Republicans on the committee pressed Wray hard over allegations that pro-Clinton, anti-Trump FBI investigators and lawyers controlled both last year's probe into Clinton's use of classified material on a private email server, and special prosecutor Robert Mueller's ongoing investigation into possible collusion between the Trump campaign and Moscow to tilt the White House race.

Trump and Republicans appear to be building such a case to undermine the Mueller probe, which has already unleashed charges against four former Trump aides and increasingly menaces the White House and Trump's family.

They focused their attacks on the high-level involvement in both the Clinton and Mueller probes of FBI agent Peter Strzok.

News reports last weekend said Strzok had been fired by Mueller as lead investigator in August over pro-Clinton text messages he sent to a Justice Department lawyer.

According to reports, he also had a major role in Comey's decision in July 2016 -- in the middle of a tense election campaign -- to clear Clinton of any crime related to her email use.

Trump seized on the reports about Strzok in his weekend anti-FBI, anti-Comey tweetstorm. 

"Report: 'ANTI-TRUMP FBI AGENT LED CLINTON EMAIL PROBE' Now it all starts to make sense!" Trump tweeted Sunday morning.

- Republicans press over investigation 'bias' -

Since then, Republicans in Congress have latched on to the Strzok case as they pressed the FBI and the Justice Department over the independence of their investigations.

In a letter to Wray on Tuesday, Senator Charles Grassley demanded information on Strzok's role in Comey's decision not to prosecute Clinton, his role in the early Russian collusion investigation, and any communications he made with negative statements about Trump or positive ones on Clinton.

In Thursday's hearing, Republicans alleged that Muller, after taking on the Russia probe in May, assembled a team of largely Democratic lawyers and investigators.

"If you kicked everybody off Mueller's team who are anti-Trump, I don't think there would be anybody left," said Republican Congressman Jim Jordan. 

Asked Thursday whether Trump truly believes the FBI under Wray is in "tatters," White House spokeswoman Sarah Sanders stressed Trump's problem was with former director Comey and people involved in the Clinton probe.

"We certainly felt like some of the political leadership at the FBI was problematic. We are glad that Director Wray is there. We feel he's going to clean up some of the messes left behind by his predecessor," she said.