The special counsel investigating Russia's election meddling is now reportedly looking at whether US President Donald Trump attempted to obstruct justice by firing the head of the FBI.

The Washington Post, citing several people who have knowledge of the investigation, said special counsel Robert Mueller's probe widened following James Comey's abrupt dismissal in early May.

Mr Comey has publicly accused Mr Trump of firing him to undermine the FBI's investigation into possible collusion between Mr Trump's 2016 presidential campaign team and Russia.

The White House quickly denied this was the reason, before Mr Trump himself confirmed it was true.

"I just fired the head of the FBI. He was crazy, a real nut job," Mr Trump told Russian officials when they visited the day afterwards, an official told the New York Times. "I faced great pressure because of Russia. That's taken off."

It's not clear how many people have been questioned by the FBI or the special counsel, or if Mr Trump will invoke executive privilege for himself or his officials to avoid answering questions - which could be illegal anyway, according to Supreme Court rulings made during the Watergate scandal.