Pentagon chief of staff quits after Mattis

Sweeney released a statement not long after a resignations was submitted by Secretary of Defense James Mattis.

Mattis's former deputy, Patrick Shanahan, began serving as acting Defense secretary at the beginning of January.

"After two years in the Pentagon, I've decided the time is right to return to the private sector. It has been an honor to serve again alongside the men and women of the Department of Defense," he said.

Before him, Pentagon spokeswoman Dana White had announced her resignation at the end of December.

The US president pushed James Mattis out of his post after disagreement over Trump's decision to end the US military presence in Syria and scale it back in Afghanistan, making Trump further isolated at helm.

Before leaving Pentagon, Mattis ordered withdrawal of an estimated 2,200 US forces from Syria.

Mattis submitted his letter of resignation after he failed to convince the president that the US military presence in Syria is associated with maintaining critical alliances and partnerships.

The long list of dismissals and resignations in the Trump administration reveals the chaotic play of power inside the White House.