The United States said Thursday it would require "concrete action" by North Korea before resuming six-party talks aimed at ending the Stalinist state's diplomatic isolation.

The US comments from State Department spokesman Mark Toner came two days after North Korean officials told a visiting Russian envoy that it was willing to return to the six-party nuclear talks "without any precondition."

"There are steps. We encourage more steps," Toner said.

"We want to see an improvement in the dialogue between South Korea and North Korea, and then as we move forward, we can begin to address other issues but... we're still looking for concrete action on the part of North Korea that they can take to basically address their previous behavior and show that their behavior has changed."

Six-party talks grouping the two Koreas, Japan, Russia, the United States and China have been deadlocked since Pyongyang walked out in April 2009 in protest at UN condemnation of an apparent missile test.

The aim of the talks is to obtain the communist North's denuclearization in return for economic aid, diplomatic recognition and the establishment of a permanent peace regime.

South Korea earlier Thursday rejected the offer from North Korea, saying its neighbor must first show peaceful intentions.

Pyongyang triggered security fears in November when it disclosed an apparently functional uranium enrichment plant to visiting US experts.

The North said it was a peaceful energy project, but experts said the facility could hand Pyongyang a second route to making atomic bombs in addition to its existing plutonium stockpile.

Russia, along with Washington and Tokyo, has backed Seoul's call for the UN Security Council to debate Pyongyang's uranium program.