US troubleshooter Bill Richardson said Tuesday that North Korea had moved in the "right direction" toward easing tensions with the South but now needed to back up that movement with "deeds, not words".

The New Mexico governor was speaking at Beijing's main airport upon arrival from Pyongyang, where he has been since Thursday on a trip aimed at restoring calm following the North's bombardment of a South Korean island last month.

"My sense is the North Koreans realise that they have moved too negatively against negotiations, that they have taken some very bad steps and they wanted to move in the right direction," Richardson told reporters.

"They have shown a certain pragmatism by not proceeding with retaliation," he said, referring to Pyongyang's decision not to react following the South's live-fire military drill on Yeonpyeong island on Monday.

"They agreed to the proposals that I made.... Now there has to be deeds, not words," said Richardson, a former US ambassador to the United Nations and veteran negotiator with the hardline regime of Kim Jong-Il.

"Now is the time for all sides to remain calm."

Richardson had announced on Monday that North Korea had agreed to allow UN nuclear inspectors back into the country and to consider the creation of a three-way military commission with Seoul and the US to monitor disputed areas.

Pyongyang also agreed to "negotiate a deal for a third party, such as South Korea, to buy fresh-fuel rods from North Korea," Richardson's office had announced.

"The specifics are that they will allow IAEA (International Atomic Energy Agency) personnel to go to Yongbyon to ensure that they are not processing highly enriched uranium and are proceeding with peaceful purposes," he said on Tuesday.

North Korea in April 2009 pulled out of six-nation nuclear disarmament talks and ordered US and UN nuclear inspectors out of the country. It staged its second nuclear test a month later.

Pyongyang then sparked global alarm last month with revelations of an extensive uranium enrichment programme.

The US State Department expressed scepticism over the concessions apparently won by Richardson, with spokesman Philip Crowley saying: "North Korea talks a great game. They always do. The real issue is what will they do."

Richardson, a high-profile Democratic politician who served as energy secretary under former US president Bill Clinton, said he met with top leaders from the foreign ministry and the military.

The New Mexico governor was invited to Pyongyang by the North's chief nuclear negotiator Kim Kye-Gwan.

"We all agreed that my visit came among the highest tensions since the Korean war," Richardson said, commending the South for its "restraint" in carrying out Monday's drill and the North for opting not to respond.

"This is the first time there has been positive movement in easing tensions. But that has to continue," he said.

"The fact that they didn't retaliate, the fact that they agreed to IAEA monitoring, the fact that they are willing to sell the fuel rods that could be used to manufacture nuclear weapons, the fact that they are willing to sell them to South Korea -- let's use that opportunity for all sides to start talking."