WASHINGTON (AFP) - Special counsel Robert Mueller is investigating whether US President Donald Trump's tweets constitute an attempt to obstruct justice with respect to the probe into his campaign's ties with Russia, The New York Times reported on Thursday (July 26).

Citing three unnamed people briefed on the subject, the Times said that Mueller, a former FBI director, was scrutinising tweets and negative statements from the president about Attorney-General Jeff Sessions and Mueller's FBI successor James Comey.

Trump has repeatedly turned to Twitter to try to bend proceedings and public opinion in the direction he would like - calling for Mueller's probe to end and demanding that the Justice Department investigate his political enemies.

In one of many similar tweets, Trump on April 20 appeared to try to undermine Mueller's probe into whether the Trump campaign colluded with Russia in its efforts to skew the 2016 US presidential election.

"James Comey illegally leaked classified documents to the press in order to generate a Special Council (sic)? Therefore, the Special Council was established based on an illegal act? Really, does everybody know what that means?" he wrote.

Now Mueller is reportedly investigating whether that pattern of behaviour is enough to constitute obstruction of justice.

Obstruction is difficult to prove, and US legal opinion is divided on whether the president can be indicted for such a crime even if it is a slam dunk case.

To avoid that legal thicket, Richard Nixon was named an unindicted co-conspirator in the Watergate scandal, which nonetheless led to his impeachment by Congress and exit from office.