Rare White House briefing on North Korea for the entire US Senate

South Korean and US officials have feared for some time that a sixth North Korean nuclear test could be imminent. Photo / AP

US President Donald Trump says the UN Security Council must be prepared to impose new sanctions on North Korea as concerns mount that it may test a sixth nuclear bomb as early as tomorrow.

"The status quo in North Korea is also unacceptable," Trump told a meeting with the 15 UN Security Council ambassadors, including China and Russia, at the White House.

"The council must be prepared to impose additional and stronger sanctions on North Korean nuclear and ballistic missile programmes."

He added: "This is a real threat to the world, whether we want to talk about it or not. North Korea is a big world problem and it's a problem that we have to finally solve. People put blindfolds on for decades and now it's time to solve the problem."

US officials have told Reuters tougher sanctions could include an oil embargo, banning North Korea's airline, intercepting cargo ships and punishing Chinese banks and other foreign doing business with Pyongyang.

The State Department said US Secretary of State Rex Tillerson would chair a special ministerial meeting of the Security Council on North Korea on Saturday NZT to discuss ways to maximise the impact of existing sanctions and show "resolve to respond to further provocations with appropriate new measures".

Tillerson and US Secretary of Defence Jim Mattis, Director of National Intelligence Dan Coats and General Joseph Dunford, chairman of the Joint Chiefs of Staff, will also hold a rare briefing on North Korea at the White House on Thursday for the entire US Senate, Senate aides said.

The White House said Trump and German Chancellor Angela Merkel discussed the "urgent security challenge" posed by North Korea in a phone call yesterday. In an earlier phone conversation with Trump, Chinese President Xi Jinping called for all sides to exercise restraint, as Japan conducted exercises with a US aircraft carrier strike group headed for Korean waters.

South Korean and US officials have feared for some time that a sixth North Korean nuclear test could be imminent and speculation has grown that this, or another missile test, could coincide with the 85th anniversary of the foundation of North Korea's army.

Trump has vowed to prevent North Korea from being able to hit the United States with a nuclear missile and has said all options are on the table, including a military strike, although officials say tougher sanctions are the preferred route.

- Reuters, AAP