WASHINGTON: Donald Trump derided claims of collusion between Moscow and his associates as "phony" Thursday (Jun 15) after reports of the head of a probe into Russia's alleged election meddling is investigating him for obstructing justice.

"They made up a phony collusion with the Russians story, found zero proof, so now they go for obstruction of justice on the phony story. Nice," Trump wrote.

They made up a phony collusion with the Russians story, found zero proof, so now they go for obstruction of justice on the phony story. Nice — Donald J. Trump (@realDonaldTrump) June 15, 2017

Senior US intelligence officials have agreed to be interviewed by investigators working for Robert Mueller, a respected former FBI chief, according to reports in both The Washington Post and New York Times.

Mueller's office has also asked the National Security Agency (NSA) for documents related to its interactions with the administration as part of the probe into Russian meddling and possible collusion with Trump campaign officials, The New York Times reported.

Mueller has the authority to investigate whether Trump tried to obstruct justice which is a potentially impeachable offense.

Former FBI director James Comey told Congress last week that Trump pressured him to drop an investigation into his former national security advisor, Mike Flynn, who was forced to quit for lying about his links to Moscow.

Comey was later sacked, with Trump acknowledging that the Russian investigation was on his mind when he decided to dismiss him.

"You are witnessing the single greatest WITCH HUNT in American political history - led by some very bad and conflicted people! #MAGA," Trump tweeted later Thursday morning. The acronym is a reference to his campaign slogan, Make America Great Again.

Trump was forced to fire Flynn in February after he misled the White House about his contacts with Russian officials last year as a member of Trump's transition team.

The president vehemently denies any collusion between himself or any of his associates and Russia.

The Post quoted five people briefed on the requests, saying those who agreed to be interviewed include national intelligence director Daniel Coats, Admiral Mike Rogers who heads the NSA, and his recently departed deputy, Richard Ledgett.

The shift toward investigating the president began days after Trump fired James Comey as FBI director on May 9, the Post said.

Comey, who had been overseeing the bureau's Russia investigation, says he had previously assured Trump he was not personally under investigation.

Recent reports have said Trump also requested help from Coats and Rogers to pressure Comey to end the investigation into Flynn's activities.