U.S. officials were looking for concrete signs that nuclear-armed North Korea is willing to take "irreversible steps" to give up its nuclear weapons programs as the two sides headed into a second day of talks Friday.

Ambassador Stephen Bosworth, the Obama administration's top envoy on North Korean affairs, and North Korean Vice Foreign Minister Kim Kye Gwan remained silent after meeting behind closed doors on Thursday at the U.S. Mission to the United Nations for a total of about five hours.

The State Department called the talks "serious and business-like" and said in a statement they will resume Friday morning.

The U.S. wants to determine if North Korea is ready to fulfill its commitments under a 2005 joint declaration requiring the North to abandon all nuclear weapons programs and allow the return of international weapons inspectors. In exchange, Pyongyang would get better relations with its Asian neighbors, energy assistance, and a pledge from Washington that its troops won't attack the North.

The statement said that Washington wants to see a willingness from the North "to take concrete and irreversible steps."

"They've been down this road before and it's a chance for us to gauge their seriousness," State Department spokesman Mark Toner said earlier Thursday. "What we're looking for is a concrete indication that they're going to move forward."

Bosworth greeted Kim at the entrance to the U.S. Mission in the shadow of the U.N. headquarters complex when he arrived with his delegation Thursday morning. They smiled and shook hands before a throng of photographers, cameramen and reporters.

Ri Gun, the director general of the North American affairs bureau in North Korea's Foreign Ministry, when asked whether he was optimistic about the meeting, replied: "I'm not sure yet."

The high-level meetings have raised hopes of a breakthrough in resuming disarmament negotiations two years after North Korea walked away from the talks, vowing never to return.

Since then, Pyongyang has expressed a willingness to rejoin the talks. Seoul and Washington, however, have insisted that inter-Korean ties must improve first in the wake of two attacks last year that killed 50 South Koreans.

The discussions aim to build on last week's surprise talks between nuclear negotiators from North and South Korea in Indonesia, the first such meeting since disarmament talks were last held in December 2008. The arms talks collapsed shortly afterward.

Secretary of State Hillary Rodham Clinton invited the North Korean vice foreign minister to New York after what U.S. officials described as a constructive meeting. But she said the U.S. wouldn't reward the North for just returning to the table or promising to uphold old agreements.

Scott Snyder, a Washington-based Koreas specialist for the Council on Foreign Relations, said there is much skepticism about the sincerity of the North Koreans, who have conducted a second nuclear test and revealed a uranium enrichment facility that could give it another way to make atomic bombs since the last talks in 2008.

Nevertheless, Snyder told AP, the talks are "significant, in that it is the first time in a while to exchange views directly."

Former U.S. ambassador to the U.N. John Bolton, now at the conservative American Enterprise Institute, was pessimistic.

"I think these talks are pointless," he told AP. "There is simply no evidence and there hasn't been for 10 years that the North Koreans are really prepared to give up their nuclear weapons program. They have used negotiations to buy time, to buy legitimacy, and to extract tangible assistance from the United States."

"In return, the United States has gotten nothing. So I'm not optimistic," Bolton said.

Toner said the U.S. has made no decisions on providing food aid, which the North is desperately seeking. North Korean officials have agreed to strict monitoring conditions _ a rare concession _ in their pleas for help, citing rising global food prices, shortfalls in fertilizer and the winter freeze that killed their wheat harvest.

Five nations _ the United States, China, Japan, South Korea and Russia _ have been negotiating since 2003 to persuade North Korea to dismantle its nuclear programs. Pyongyang pulled out of the six-party talks in April 2009 after being censured for launching a long-range rocket.

However, North Korea and China have made recent calls to resurrect the negotiations.

Kim told reporters after landing in New York on Tuesday that he was "optimistic" the six-party talks could resume and that relations with the U.S. might improve.

"Now is the time for countries to reconcile," he said, according to South Korea's Yonhap News Agency.

On Wednesday, Pyongyang pressed for the U.S. to sign a peace treaty formally ending the Korean War. In an editorial marking the 58th anniversary of an armistice ending the 1950-53 conflict, the official Korean Central News Agency repeated a long-held demand of the North Korean government, saying a peace treaty could help resolve the nuclear deadlock.

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Associated Press Writers Anita Snow at the United Nations and Matthew Lee contributed to this report.