A US submarine has arrived in South Korea, amid worries of another North Korean missile or nuclear test.

The missile-armed USS Michigan is set to join an incoming group of warships led by aircraft carrier Carl Vinson.

North Korea is celebrating its army's 85th founding anniversary on Tuesday. It has previously marked similar occasions with missile tests.

Tensions have risen on the Korean peninsula in recent weeks as the US and North Korea exchange heated rhetoric.

Meanwhile in an unusual event, the entire US Senate has been asked to attend a briefing on North Korea on Wednesday at the White House.

The USS Michigan is a nuclear-powered submarine carrying 154 Tomahawk cruise missiles and 60 special operations troops and mini-subs, reported the South Korean newspaper Chosun Ilbo.

It is expected to take part in military exercises with the Carl Vinson warship group dispatched by the US in a show of force.

The warships were briefly at the centre of some confusion when they did not immediately head to the Korean peninsula, but US Navy officials said it is now proceeding to the region as ordered.

Pyongyang has reacted angrily, threatening to sink the aircraft carrier and to launch a "super-mighty pre-emptive strike" against what it calls US aggression.

US President Donald Trump said earlier this month in a TV interview he was sending an "armada" and that the US had submarines which are "very powerful, far more powerful than the aircraft carrier".

China has repeatedly urged for calm, with President Xi Jinping speaking to Mr Trump on Tuesday urging all parties to "maintain restraint and avoid actions that would increase tensions".