The US is discussing with China imposing further sanctions against North Korea as the reclusive country is "not even close" to taking steps to rein in its nuclear weapons programme, Secretary of State John Kerry has warned.

Speaking in the South Korean capital, Mr Kerry said Washington had offered the North the chance of an improved relationship in return for signs of a genuine willingness to end its nuclear programme.

"To date, to this moment, particularly with recent provocations, it is clear the DPRK is not even close to meeting that standard," Mr Kerry told a joint news conference with South Korean Foreign Minister Yun Byung-se.

"Instead it continues to pursue nuclear weapons and ballistic missiles."

North Korea is already under heavy UN, EU and US sanctions for its missile and nuclear tests.

"I think never has the international community been as united as we are now, that, number one North Korea needs to denuclearize," Mr Kerry said, adding a pending nuclear deal with Iran could serve as an example to the North.

"With respect to the methodology for boosting sanctions and other things, we (the United States and China) are discussing all of that now. China has obviously an extraordinary leverage.

"We will have security and economic dialogue with the Chinese in Washington in June and that will be the moment where we will table some of these specific steps."

Mr Kerry did not elaborate on possible measures and it was not clear whether he was referring to steps that would be taken by the two powers or by the UN.

China has a history of resisting tough moves against North Korea not mandated by resolutions of the UN Security Council, on which it sits with veto power.

Pyongyang walked away from a 2005 deal with China, Japan, Russia, South Korea and the United States to end its nuclear programme in return for diplomatic and economic rewards.