WASHINGTON • The campaign to win China's backing for deeper sanctions against North Korea has gained in intensity, with the US and South Korean presidents making their case directly to Chinese President Xi Jinping.

Presidents Barack Obama and Park Geun Hye spoke to Mr Xi by phone in separate calls on Friday to demand punitive measures following a recent nuclear test by the North.

On Jan 6, Pyongyang detonated what it claimed was a powerful thermonuclear bomb. It has followed up by threatening to launch a satellite-bearing rocket - an operation widely seen as a covert ballistic missile test.

The White House and its allies want to respond with a United Nations resolution that would slap more sanctions on the North. But they must first win the backing of UN veto power China, which has shielded its neighbour in the past.

The White House said that Mr Obama and Mr Xi agreed on the need for a "strong and united international response to North Korea's provocations", including "through an impactful UN Security Council resolution", But the terse White House statement did not indicate whether that meant agreement on specific steps.

Mr Xi received a similar message late on Friday from Ms Park, who has spent political capital to improve relations with Beijing.

During a 45-minute phone call, "President Park stressed strong and effective resolutions that could force North Korea to change its course must be adopted at the UN Security Council this time", Seoul's Blue House said in a press statement.

"In this context, President Park called for active cooperation from China", which has "leverage with various means" over the North, the statement added.

China's official Xinhua state news agency said Mr Xi stressed China was firmly committed to the denuclearisation of the Korean Peninsula and insists on a solution through dialogue and consultation.

AGENCE FRANCE-PRESSE