Washington:  A US navy strike group will be moving towards the western Pacific Ocean near the Korean peninsula as a show of force as concerns grow about North Korea's advancing weapons program.

Earlier this month North Korea tested a liquid-fuelled Scud missile which travelled only a fraction of its range.

The strike group, called Carl Vinson, includes the aircraft carrier Carl Vinson and will make its way from Singapore toward the Korean peninsula, according to a naval official, who was not authorised to speak to the media and requested anonymity.

"We feel the increased presence is necessary," the official said, citing North Korea's concerning behaviour.

President Donald Trump and Japanese Prime Minister Shinzo Abe have agreed that the two nations and South Korea must maintain unity in dealing with North Korea launching ballistic missiles.

Mr Trump and Mr Abe spoke by phone for about 45 minutes, exchanging views on the regime of North Korea's Kim Jong  Un and Syria, Japan's national broadcaster NHK reported on Sunday in Tokyo. Mr Abe said he's watching closely how China will react to the North Korea issue, according to NHK, after Mr Trump held his first meeting with Chinese President Xi Jinping in Florida.

"We completely agreed that it's very important for Japan and US to cooperate and work together closely and the unity among Japan, US and South Korea is important," Mr Abe said.

The phone conversation between Mr Trump and Mr Abe took place as a ballistic missile test by North Korea drew condemnation from countries in the region and the US missile strike in Syria increased tensions with Russia. Mr Trump also explained to Mr Abe that he conducted the missile attack in order to prevent chemical weapons from being used again after many civilians including women and children were harmed, according to Kyodo news service.

In a statement late on Saturday, the US Navy's Third Fleet said the strike group had been directed to sail north, but it did not specify the destination. The military vessels will operate in the Western Pacific rather than making previously planned port visits to Australia, it added.

This year North Korean officials, including Kim Jong Un, have repeatedly indicated an intercontinental ballistic missile test or something similar could be coming, possibly as soon as April 15, the 105th birthday of North Korea's founding president and celebrated annually as "the Day of the Sun".

Mr Trump's national security aides have completed a review of US options to try to curb North Korea's nuclear and missile programs. These include economic and military measures but lean more toward sanctions and increased pressure on Beijing to rein in its reclusive neighbour.

Although the option of pre-emptive military strikes on North Korea is not off the table, the review prioritises less-risky steps and plays down  direct military action.

Mr Trump spoke with South Korea's acting President Hwang Kyo-ahn on Friday, the White House said on Saturday.

Reuters, Bloomberg