The Trump administration is considering placing North Korea on a list of state sponsors of terrorism as US senators arrived at the White House for an extraordinary briefing on the threat posed by the rogue state.

All 100 senators were invited to a classified briefing at a building next to the White House on Wednesday (US time), reports AFP. They were ferried from Capitol Hill by bus.

President Donald Trump's secretary of state, defence secretary, top general and national intelligence director will brief them. The briefing team was to speak later to members of the House of Representatives lower chamber.

US senators prepare to board a bus on Capitol Hill in Washington for the White House briefing. (Photo: AP).

Former US president George W. Bush removed North Korea from the list of state sponsors of terrorism in 2008 but it is understood the Trump administration is now considering placing it back on.

Though highly unusual, the briefings do not necessarily reflect a security crisis is imminent, although military tensions are rising on the divided Korean Peninsula.

But alarm is rising over North Korea's progress in developing a nuclear-tipped missile that could strike America.

Earlier, a leading US commander told Congress a US missile defence system being established in South Korea to defend against any North Korean attack should be operational in the coming days.

US President Donald Trump and Vice-President Mike Pence walk to the walk to the Eisenhower Executive Office Building on the White House complex in Washington where senators are being briefed on North Korea. (Photo: AP).