Former CIA Director John Brennan said on Wednesday that US President Donald Trump's decision to revoke his security clearance was part of an effort to silence critics.

"This action is part of a broader effort by Mr Trump to suppress freedom of speech & punish critics," Brennan wrote on Twitter. "It should gravely worry all Americans, including intelligence professionals, about the cost of speaking out."

In an interview on MSNBC, Brennan said Trump's action, announced on Wednesday by the White House, was politically motivated.