Leading US lawmakers expressed dissatisfaction Tuesday with stalled US diplomatic efforts to dismantle North Korea's nuclear program, warning of the risks of proliferation and regional instability.

Senator John Kerry, the chairman of the Senate Foreign Relations Committee, urged the administration to seize the initiative through bilateral negotiations with Pyongyang, citing "the very real risks" posed by the North Korean programs.

"We must get beyond the political talking point that engaging North Korea is somehow 'rewarding bad behavior.' It is not," Kerry said.

North Korea has repeatedly pressed the United States to hold direct talks on ending their nuclear standoff, but Washington has so far preferred a multilateral approach, involving China, South Korea and others.

Senator Richard Lugar, the ranking Republican, questioned whether President Barack Obama had a viable strategy for dismantling North Korea's nuclear program, and whether sufficient priority had been given to it in US relations with China.

"While the administration has worked closely with South Korea in response to various North Korean provocations during the last two years, it is less clear that the administration has developed a strategy with the potential to dismantle North Korea's nuclear weapons program," he said.

Ambassador Stephen Bosworth, the US pointman on North Korean issues, and Kurt Campbell, assistant secretary of state for East Asia, defended the administration's refusal to resume talks now with North Korea, even in a multilateral forum.

"We want assurance that North Korea regards these prospective talks seriously. We are not interested in talking for the sake of talking," said Bosworth. "We want to talks which produce concrete results."

Bosworth said the administration remains "open to constructive dialogue" but North Korea first must show it is serious about agreements it made in earlier negotiations.

North Korea quit so-called six party talks involving the United States, South Korea, Russia, China, and Japan in April 2009.

A month later it staged its second nuclear test. Tensions have soared over the past year with the sinking of a South Korean warship in March 2010, the shelling of a South Korean border island in November, and the disclosure of the existence of a secret uranium enrichment program.

The US military has warned there may be further "provocations," amid unconfirmed reports of activity at North Korean missile and nuclear test sites.

Bosworth said the existence of the uranium enrichment program, which the North showed off to a visiting American expert, was not a surprise but had complicated prospects for negotiating a de-nuclearization agreement with North Korea.

"We know that the centrifuges are there. We cannot verify that they are operating, and we cannot verify any production of enriched uranium," he said.

Meanwhile, Campbell acknowledged there was more that could be done to tighten enforcement of sanctions against Pyongyang, calling North Korean weapons proliferation "an enormously challenging problem."

He highlighted US concerns about North Korean proliferation to Burma.

"In the past, most North Korean proliferation activities have affected the Middle East. But in the recent period they have increased substantially (in Burma) -- the provision of certain conventional technologies, small arms and also some missile components to Burma, in clear violation of UN Security Council resolution," he said.

At the same time, Bosworth said the United States was giving no consideration to re-introducing nuclear weapons in South Korea as a deterrent.

"Our mission in South Korea is to deter any aggression by North Korea, and we are very confident that we have more than adequate tools at our disposal to accomplish that mission of deterrence," Bosworth said. "This is not an issue that is under active consideration."