In an unusual move, the entire US Senate is being called to the White House for a briefing on North Korea.

Washington has become increasingly concerned at North Korean missile and nuclear tests and threats to its neighbours and the US.

The briefing, involving 100 senators as well as Secretary of State Rex Tillerson and Defence Secretary James Mattis, is being held on Wednesday.

China, North Korea's main ally, has called for restraint from all sides.

China's call came in a phone conversation between President Xi Jinping and President Donald Trump on Sunday.

Mr Xi urged all parties to "maintain restraint and avoid actions that would increase tensions", according to the Chinese foreign ministry.

For his part, Mr Trump said North Korea's "continued belligerence" was destabilising the Korean peninsula.

White House officials regularly go to Congress to brief on national security matters, but it is unusual for the whole Senate to go to the White House.

Alongside Mr Tillerson and Gen Mattis will be National Intelligence Director Dan Coats and Gen Joseph Dunford, chairman of the Joint Chiefs of Staff.

Questioned by reporters at his regular briefing, White House Press Secretary Sean Spicer referred further inquiries to the Senate majority leader, Mitch McConnell.

Aides, quoted by Reuters, say the House of Representatives is seeking a similar briefing on North Korea.

Washington says a flotilla, led by the aircraft carrier USS Carl Vinson, is expected to arrive off the peninsula within days, despite conflicting messages last week on its exact whereabouts.

Mr Trump has also told United Nations Security Council ambassadors, meeting at the White House, that the UN must be ready to impose new sanctions on North Korea.

The US is pursuing a multi-pronged strategy to make its point on North Korea, the BBC's Gary O'Donoghue in Washington says.

First it wants the UN to tighten sanctions even further, and to ensure those in place are properly enforced.

Secondly, it is attempting to put fresh pressure on China to rein in its communist neighbour. The third aspect is the sending of an aircraft carrier to the Korean peninsula - making clear military action is an option.

North Korean rhetoric

North Korean state media said on Sunday the country's forces were "combat-ready to sink" the Carl Vinson.

The ruling Workers' Party newspaper Rodong Sinmun called the carrier a "gross animal".

North Korea has promised to press ahead with missile tests despite Mr Trump's warnings and experts say it may be preparing for another nuclear test, in defiance of UN resolutions.

However, a North Korean ballistic missile test on 16 April failed within seconds of launch, US experts said.

Washington is concerned that Pyongyang may develop the ability to place a nuclear bomb on a missile capable of reaching the US.

Beijing is worried about the possibility of all-out conflict on the peninsula, which could lead to the collapse of the North Korean regime under its mercurial leader Kim Jong-un.

China fears this could cause a sizeable refugee problem and lead to an American presence up to the Chinese border.