The United States and North Korea (search) met privately for the second straight day Thursday, escalating their dialogue on the sidelines of six-country talks to resolve a stalemate over the North's nuclear program.

"They had bilateral talks with the United States for about 1 1/2 hours yesterday afternoon and I am aware that they met briefly again this morning before the plenary session," Jeong Se-hyun, South Korea's unification minister, told reporters in Seoul.

The U.S. Embassy in Beijing said it had no immediate information on the new meeting. It was unclear whether the chief delegates of the two countries, who met in the first one-on-one talks Wednesday, were involved.

South Korea's (search) offer for North Korea to be compensated if it relinquishes its nuclear programs was on the table as the six-country talks reconvened Thursday, with the United States pushing for a verifiable end to the North's ambitions of becoming a nuclear power.

Delegates began the second day of negotiations emphasizing that any conclusions were premature. "It's just getting started," Japan's delegate, Foreign Ministry Director General Mitoji Yabunaka, said before the talks reconvened Thursday.

The second day of meetings among the two Koreas, China, Russia, Japan and the United States on the nuclear standoff followed a rare, lengthy one-on-one session between high-level officials from Washington and Pyongyang (search) -- the two key players in the dispute.

Neither side gave details of the meeting Wednesday afternoon between U.S. Secretary of State James Kelly and North Korean Vice Foreign Minister Kim Kye Gwan, but the U.S. State Department described it as "useful."

North Korea and the United States have been at odds over Pyongyang's nuclear ambitions for years and especially since October 2002, when Kelly said the North told him it had a secret weapons program based on enriched uranium.

North Korea publicly denies it has a uranium program in addition to its known plutonium-based program, but it brandishes the threat of what it vaguely describes as its "nuclear deterrent" in an effort to extract concessions.

The impoverished North wants aid in return for halting its nuclear programs, and in December demanded economic aid and other U.S. concessions in return for a freeze. Washington said at the time that Pyongyang must not only freeze, but start dismantling, its nuclear programs first.

North Korea also wants a nonaggression treaty with the United States or at least a security guarantee from all five of its negotiating partners.

During the opening of talks Wednesday, Kelly said Pyongyang has nothing to worry about. The United States wants an end to all of the North's nuclear weapons development but has "no intention of invading or attacking" the country, he said.

"The United States seeks complete, verifiable and irreversible dismantlement of all North Korea's nuclear programs, both plutonium and uranium," Kelly said.

North Korea reiterated demands for compensation ahead of the talks, and South Korea proposed "countermeasures" if the North froze its nuclear program and showed signs of scrapping it.

"If it is such a freeze, we can push for countermeasures," Seoul's head delegate, Lee Soo-hyuck, told reporters. He didn't elaborate, and it was unclear whether the United States had directly endorsed the proposal.

This week's meeting is the second round of six-party talks. The first one in August, scheduled for three days only, yielded little more than a vague promise to meet again. Parties have made this meeting open-ended, hoping for more progress.

"I think it's realistic optimism," said Bill Tow, a professor of international relations at Griffith University in Brisbane, Australia. "They wouldn't have come together at this juncture unless they felt there was a reasonable chance there might be some progress made."

All of North Korea's partners in the talks say they want a nuclear weapons-free Korean Peninsula.