BEIJING (AFP) - US Secretary of State John Kerry met top officials of North Korea's key ally and aid provider China on Saturday to press them to rein in a defiant Pyongyang, seeking Beijing's help to defuse soaring nuclear tensions.

Mr Kerry met first with China's foreign minister Wang Yi after flying in from talks in South Korea with President Park Geun Hye, where he offered public US support for her plans to initiate some trust-building with the North.

The Korean peninsula has been engulfed by escalating military tensions and dire threats of nuclear war ever since North Korea conducted a rocket test last December and a nuclear test in February.

"Obviously there are enormously challenging issues in front of us, and I look forward to having that conversation with you today," Mr Kerry told Mr Wang.

Mr Wang agreed the visit came at a "critical moment".

China has backed North Korea since the 1950-53 Korean War and could wield tremendous leverage over the isolated communist regime thanks to the vital aid it provides, including almost all of its neighbour's energy imports.

But analysts say it is wary of pushing too hard for fear of destabilising North Korea, which could send a wave of hungry refugees flooding into China and ultimately lead to a reunified Korea allied with the United States.

Mr Kerry will meet China's new President Xi Jinping and Premier Li Keqiang later on Saturday.