The chief U.S. nuclear negotiator arrived in North Korea Wednesday carrying a compromise aimed at salvaging a derailed nuclear disarmament pact, officials said.

Assistance Secretary of State Christopher Hill crossed into the North through the heavily fortified Demilitarized Zone dividing the two Koreas, said U.S. Embassy spokesman Aaron Tarver.

His trip comes with Pyongyang growing increasingly defiant toward international efforts to convince the communist state to eliminate its nuclear program. Since mid-August, the North has begun to restore its nuclear facilities and ordered U.N. nuclear monitors to leave the country in violation of an international accord.

Hill said Tuesday night that his goal was to persuade North Korea's Vice Foreign Minister Kim Kye Gwan to agree to Washington's demand for a verification system to account for the North's nuclear arsenal. But he acknowledged it would be a difficult task.

The North has rejected U.S. requests on verification and accused Washington of not living up to its end of the deal and removing North Korea from a list of state sponsors of terrorism. It recently reversed the process of dismantling its nuclear facilities.

"We are in a very difficult, very tough phase of negotiations," Hill told reporters Tuesday night after meeting with his South Korean counterpart, Kim Sook, to discuss ways to persuade the North to return to the disarmament process.

In Washington, a senior U.S. official said Hill is bringing a new face-saving proposal that would have North Korea agree to a verification program and submit it first to its Chinese allies. The official spoke on condition of anonymity because Hill has not presented the proposal.

The U.S. would then provisionally remove North Korea from the terrorism sponsors list. That would edge around the current impasse, in which the U.S. says it won't remove North Korea from the list until it signs up to the verification measures.

U.S. officials said they were not sure North Korea will agree to the idea and if they do, whether what they present to the Chinese will be acceptable to Washington.

Hill's trip to the capital, Pyongyang, comes amid reports that autocratic North Korean leader Kim Jong Il suffered a stroke in August, prompting concern that his prolonged illness could destabilize the Korean peninsula. North Korea denies that Kim, 66, is ill.

Kim's disappearance from the public eye coincided with an about-face on the 2007 nuclear deal painstakingly negotiated among six countries — the two Koreas, the U.S., China, Russia and Japan.

North Korea alarmed the world in 2006 by testing a nuclear device and a series of missiles, including one capable of reaching as far as Alaska. It then agreed to dismantle its nuclear program in exchange for energy aid and other concessions.

The regime began disabling its nuclear processing plant in Yongbyon in November, and blew up a cooling tower in June in a dramatic display of its determination to carry out the process.

Just steps away from completing the second phase of the three-part process, Pyongyang abruptly reversed course in mid-August and stopped disabling the plant.

After confirming it had begun restoring the nuclear reprocessing plant and testing an engine ignition, the regime last week ordered U.N. nuclear inspectors to leave the country and said it planned to restart the plant. Experts say it could be up and running within months.

In a further sign of defiance, South Korea's Yonhap news agency reported Wednesday that the North is showing signs of restoring the site where it conducted its nuclear test blast.

Yonhap cited an unidentified government official saying that Seoul has detected smokes from the Punggyeri site in the country's northeast, and that could be a sign the area is undergoing a possible restoration.

South Korea's Defense Ministry said it could not confirm the report.

"What they have been doing obviously goes counter to the spirits of what we've trying to accomplish because all of the disablement — shutdown and disablement — was for the purpose of abandonment" of its nuclear program, Hill said.

At issue is Washington's request that the North agree to a verification system to account for its nuclear arsenal as a condition for removing North Korea from the list of terrorism sponsors.

The detailed, four-page outline of the verification process that Washington seeks calls for a thorough inspection, soil samples, interviews with scientists and possible involvement of the United Nations' nuclear agency.

Notoriously reclusive North Korea objects to having to prove its declaration of nuclear facilities, saying verification was never part of the disarmament-for-aid deal and is a unilateral move to disarm them.

U.S. officials say the proposed verification is standard and has been used by other countries to account for their nuclear programs.

The U.S. official in Washington said Hill plans to go word-by-word through the proposed verification protocol to try to address the North's concerns although previous such efforts did not work.

"I know they are reluctant. ... Let's sit down and have a conversation and see if we can resolve this matter," Hill said earlier Tuesday, before his meeting with the South Korean negotiator.

The trip to Pyongyang will be Hill's third to North Korea since December. He is scheduled to drive into the North from the heavily fortified Demilitarized Zone on Wednesday, officials said.

Pyongyang, about 60 miles from the border that divides the two countries still technically at war, is several hours' drive from the DMZ.