Beijing: China and South Korea have agreed to take new measures against North Korea if the rogue state conducts another nuclear or intercontinental missile test, Chinese media has reported.

As concern mounts that North Korea will use a national commemoration on Saturday to conduct its sixth nuclear test, claims the Chinese army has amassed 150,000 troops including medical teams on the border with North Korea have been repeated on the front page of China's state-owned Global Times newspaper.

North Korea's Supreme People's Congress began its annual sitting on Tuesday, and there is speculation a nuclear plan will be unveiled ahead of April 15, the birthday anniversary of North Korean founder Kim Il-sung.

North Korea's daily newspaper warned on Tuesday: "Pre-emptive strike is not an American monopoly".

North Korea's newspaper, the Rodong Sinmun, on Tuesday carried a front page message of congratulations from Syrian President Bashar al-Assad to Kim Jong-un, on his fifth anniversary as Supreme Leader.

His sister, Kim Yo-jong, emerged publicly for the first time in nine months on Monday and was photographed with members of the Congress laying floral tributes at a statue of Kim Il-sung, her grandfather.

China's foreign ministry spokeswoman on Monday said she was "not aware" of the South Korean reports of a Chinese troop build-up.

But the Global Times said the reports were being widely circulated, and cited South Korean diplomats as saying that while the claims may be slightly exaggerated, it was true that military drills were conducted in the border area.

The People's Liberation Army has recently expressed concern that northern Chinese towns were at risk of nuclear pollution from North Korea's tests, and said that contamination would not be tolerated.

China's nuclear envoy Wu Dawei met with his South Korean counterpart Kim Hong-kyun in Seoul to discuss the North Korean crisis.

South Korea's Yonhap news agency reported the two sides had agreed that if North Korea conducts another nuclear test or missile launch, China and South Korea will carry out any future United Nations Security Council resolutions that are passed.

China's Foreign Ministry declined to comment. On Friday the Chinese government ordered Chinese trading companies to return North Korean coal cargos, to comply with UN sanctions.

South Korea's Defence Ministry confirmed the US aircraft carrier USS Carl Vinson had arrived in Korean waters in preparation for a response to a possible nuclear test.

But China has reportedly told the United States and Korea that they need to address North Korea's "security concerns" and sanctions are not enough to achieve denuclearisation.

US Secretary of State Rex Tillerson told American media on Sunday that Chinese President Xi Jinping had agreed that the situation on the Korean peninsula had intensified and action needed to be taken.

China's Foreign Ministry spokeswoman said Mr Xi and US President Donald Trump had agreed to maintain "close communication and cooperation on the issue". "All relevant parties should exercise restraint and avoid activities that may escalate the tension," she said on Monday:

The North Korean newspaper reported that the United States had pinned its hopes on a pre-emptive strike to make up for the failure of its Korea policy.

"There is something the US [should] realise, which is pre-emptive strike is not an American monopoly," the report read.

"Our armed force is ready for any means of invasion."

- with Sanghee Liu