The United States has urged China to stress the need for restraint to its North Korean allies as Pyongyang undergoes a leadership transition, a senior US diplomat said Thursday.

Assistant Secretary of State Kurt Campbell, who arrived in South Korea from talks in China, said he asked Beijing to consult closely with Washington on developments in the North.

"We also urge China to make clear the importance of restraint by the new North Korean leadership," Campbell told reporters after talks with Foreign Minister Kim Sung-Hwan and other officials.

The US also "stressed that North Korea should take steps to improve relations with the ROK (South Korea) and to abide by its international obligations and commitments", he said.

The North has harshly criticised the South's leaders since Kim Jong-Un was proclaimed "great successor" to his father Kim Jong-Il, who died on December 17.

It has threatened unspecified retaliation against Seoul for perceived insults during official mourning for Kim.

Campbell said better cross-border relations were essential in improving ties between the North and the international community.

The United States bases 28,500 troops in the South to protect it from the North. The assistant secretary said he stressed the strength of the alliance during his talks in Seoul.

"In all our meetings I underscored the strongest possible commitment of the US to the enduring partnership...and the determination to be tightly aligned as we together face the new leadership transition in North Korea," he said.

In Beijing Wednesday Campbell urged all parties to refrain from "any provocations" after Kim Jong-Il's death. He was the first US diplomat to visit China -- the North's sole major ally -- since the death.