US President Donald Trump has called for tougher new United Nation sanctions on North Korea, after the country conducted large-scale artillery exercises marking the 85th anniversary of the foundation of its armed forces.

South Korea's Yonhap News Agency said there were signs North Korea's military was carrying out large-scale, live-fire drills in areas around the city of Wonsan on its east coast today.

The Office of the South Korean Joint Chiefs of Staff said: "Our military is closely monitoring the North Korean military's movement in Wonsan areas and we are firmly maintaining readiness."

North Korea defiantly said in a state media commentary marking the 85th anniversary of the foundation of the Korean People's Army's that its military was prepared "to bring to closure the history of US scheming and nuclear blackmail."

President Trump told a meeting with the 15 UN Security Council Ambassadors at the White House that North Korea is a global threat and "a problem that we have to finally solve."

"The council must be prepared to impose additional and stronger sanctions on North Korean nuclear and ballistic missile programs," he said.

Show of force

A US submarine has docked in South Korea in a show of force amid growing concern over Pyongyang's nuclear and missile programs.

The port call by the USS Michigan submarine came as a US aircraft carrier strike group continues to head for Korean waters.

Top nuclear envoys from South Korea, Japan, and the United States were also meeting in Tokyo to discuss the North's refusal to give up its nuclear program.

Special briefing

Separately the entire US Senate has been called to a rare briefing on the perceived threat in the White House on Wednesday.

Top Trump officials including US Secretary of State Rex Tillerson, Defence Secretary Jim Mattis, Director of National Intelligence Dan Coats and Joint Chiefs chairman General Joseph Dunford will hold the briefing tomorrow.

Mr Tillerson will then chair a special ministerial meeting of the UN Security Council to discuss North Korea on Friday.

Mr Trump told the council on Monday that the “status quo in North Korea is also unacceptable."

In a phone conversation with the US president on Monday, Chinese President Xi Jinping called for all sides to exercise restraint.

Trump has sought to pressure China to do more to rein in its nuclear-armed neighbour.