The US justice department (DoJ) says it is investigating several television networks over whether they broke federal competition laws when they recently suspended pool coverage of President Donald Trump.

The agency has asked ABC, CBS, CNN, NBC and Fox News to hand over communications related to the White House TV pool, a system where networks switch off covering the president and providing the footage to other publications.

The networks temporarily suspended this system last month for several days after the president blocked CNN, MS Now and Politico from the White House.