WASHINGTON (Yonhap News) -- North Korean leader Kim Jong-il's reported promise to impose a moratorium on nuclear testing is "insufficient," as its uranium enrichment program remains a serious concern, the U.S. government said Wednesday.

Washington's cautious stance came in response to a host of media reports that the North's leader said his communist regime is ready to suspend a nuclear test and rejoin the six-way nuclear talks. He reportedly made his remarks in summit talks with Russian President Dmitry Medvedev.

"Obviously, if in fact they are now willing to refrain from nuclear test and missile launches, this would be welcome, but it would be insufficient," State Department spokeswoman Victoria Nuland said at a press briefing.

She pointed out that Pyongyang disclosed its uranium enrichment facilities last November, a clear sign that the reclusive nation is seeking the second path to develop its nuclear arsenal. The North conducted two plutonium-based weapons tests each in 2006 and 2009, prompting the U.N. Security Council to slap sanctions on it.

"As you know, their disclosure last November of uranium enrichment facilities remains a matter of serious concern to us, and these activities are a clear violation of their obligations under U.N. Security Council Resolutions 1718, 1874 and contrary to the commitments that they made in 2005," Nuland said.

In the 2005 deal at the six-party talks, the North agreed to abandon all of its nuclear program in exchange for political and economic incentives.

The nuclear talks also involve South Korea, China and Japan.

Nuland said Russia, as a member of the six-party negotiations, has the same goal as the U.S. -- to denuclearize the North.

"I think the end-state goal is the same," she said. "We've worked very hard to maintain close contacts with the Russians, and I think that we will not go back to the six-party talks until the North Koreans have prepared to meet all of the commitments that we've all laid out."