WASHINGTON (REUTERS) - US Senator Richard Burr said on Thursday (May 14) he would step aside as chairman of the powerful Senate Intelligence Committee, after the FBI seized his mobile telephone during an investigation of stock trades he made before the sharp market downturn triggered by the coronavirus pandemic.

Republican Senate Majority Leader Mitch McConnell said Burr contacted him on Thursday morning to inform of his decision to step aside temporarily during the investigation.

"We agreed that this decision would be in the best interests of the committee and will be effective at the end of the day tomorrow," McConnell said in a statement.

Burr had denied wrongdoing and said he relied solely on news reports to guide decisions on stock sales, amid reports that he and other senators sold shares after private briefings on the risks of the coronavirus crisis.

He told reporters at the US Capitol on Thursday he decided to step aside because he did not want the investigation to distract the intelligence committee from its work. "I thought this was the best thing to do," Burr said.

The Republican senator turned over his phone to Federal Bureau of Investigation agents after they served a search warrant at his Washington home.

The warrant marked a significant step-up in the investigation of Burr's stock sales in mid-February, when he and other lawmakers were getting regular briefings on the coronavirus outbreak and President Donald Trump and some of his political allies were downplaying the threat to the public.

Trump said he did not know anything about Burr's decision."I know nothing about it... That's too bad," he told reporters.

A senior Justice Department official said the FBI did not conduct a raid, but paid a visit to Burr's home to collect his cell phone. Approval of the warrant - a significant development because it was served on a sitting senator - was obtained at the "highest levels" of the Justice Department, the official said.

Reuters could not immediately learn precisely when the search warrant was executed. The seizure was first reported by the Los Angeles Times.

The Intelligence Committee chairmanship is one of the most important positions in the Senate.

As overseer of US intelligence, the committee is near completion of an extensive report on Russia's involvement in the 2016 U.S. presidential election. US intelligence determined that Moscow sought to meddle in the campaign to boost Trump candidacy.

Moscow denies such actions. Trump dismisses the allegations as a hoax.