Donald Trump has opened himself up to obstruction of justice claims by revealing he knew his former national security adviser had lied to the FBI.

In a tweet playing down the Russian links controversy, Mr Trump said he knew Michael Flynn lied to agents before he left the administration in February.

But the comment was immediately jumped on because Mr Trump allegedly encouraged the FBI to drop an investigation into Mr Flynn that same month.

Knowing Mr Flynn had lied to the FBI - a criminal offence - but still telling investigators to stop their case would amount to obstruction of justice, critics said.

Obstruction of justice is one of the reasons a US president can be impeached and was what Richard Nixon was eventually brought down over.