China must help the international community close loopholes in sanctions on North Korea after an "unprecedented" series of provocations by Pyongyang this year, a top US envoy said Tuesday.

With two nuclear tests and more than 20 missile launches so far in 2016, the North's leader Kim Jong-Un has hit a new low, said Sung Kim, the US State Department's special representative for North Korea policy.

"North Korea once again has demonstrated blatant and reckless disregard for its commitments and international obligations," Kim said during a two-day visit to Seoul.

"This is quite unprecedented, even by North Korea standards," he told journalists after meeting with his South Korean counterpart Kim Hong-Kyun.

Following Pyongyang's fourth nuclear test in January, the UN Security Council adopted the toughest sanctions regime to date, targeting the North's trade in minerals and tightening banking restrictions.

But since then, Pyongyang has carried out a series of rocket launches, and on Friday detonated its biggest ever nuclear bomb.

The UN Security council is scrambling to come up with new penalties, but five sets of UN sanctions since it first tested a nuclear device in 2006 have failed to deter the North.

The international community has also engaged in a flurry of diplomacy in an attempt to persuade China to use its leverage.

China -- Pyongyang's sole ally and largest benefactor -- had a key role to play in showing North Korea the "serious consequences for its unlawful and dangerous actions," Kim said.

Beijing has said it opposes the testing, but analysts believe it pulls its punches because it is desperate to avoid anything that would imperil the status quo and alter the balance of power on the Korean peninsula in favour of the US.

"We look forward to working with Beijing, to... try to close any loopholes" in the latest sanctions, Kim said.

Sanctions will not produce "immediate, instant success," he said, adding it would "require sustained and concerted effort...to really have the kind of effect that we desire."