BEIJING - UN nuclear monitors arrived in North Korea on Tuesday to discuss DPRK's plans to fulfill its long-delayed pledge to shut down its main nuclear reactor.

Olli Heinonen, the International Atomic Energy Agency's deputy director general for safeguards speaks to journalists before leaving his hotel in Beijing Tuesday, June 26, 2007. [AP]

International Atomic Energy Agency Director General Olli Heinonen was quoted as saying by Japan's Kyodo News agency that he was optimistic about the five-day visit after arriving in Pyongyang. North Korea's official Korean Central News Agency also confirmed the arrival.

Heinonen told reporters in Beijing earlier that the trip would focus on ways to verify that Pyongyang is taking its main reactor off-line.

The North Korean government vowed Monday to move forward with a February agreement to shut down its plutonium-producing Yongbyon reactor in exchange for aid, after announcing that a dispute over frozen bank funds that had held up disarmament efforts was now finally over.

South Korea responded Tuesday by saying it will start sending promised food aid to North Korea on June 30.

The South agreed in April to give the impoverished North 400,000 tons of rice.

Assistant Secretary of State Christopher Hill said Saturday he believed the shutdown would happen within about three weeks.

However, Heinonen told reporters before departing Beijing that it will be up to North Korea to decide how quickly the facility is shut and sealed.

"They are the ones who (will) shut it down and not us so they have to make their own plans. How long it will take is a little bit up to them," Heinonen said. "These are the details we are going to discuss now when we go there."

He said Monday he was unsure whether he would have a chance to actually visit the Yongbyon site.

Still, the visit fueled optimism that Pyongyang finally was ready to move forward with its disarmament commitments.

North Korea, which expelled UN inspectors in late 2002, announced last week that it invited a "working-level delegation" to discuss procedures for shutting down the plutonium-producing facility.

North Korea had pledged in February to shut down Yongbyon and IAEA Chief Mohamed ElBaradei traveled to North Korea in March in what was billed as a landmark visit.

But Pyongyang refused to act on the promise until it received about $25 million in funds that were frozen in a Macau bank amid a dispute with the US over alleged money-laundering.

On Monday, North Korea announced the banking dispute had been resolved and said it would start implementing the February disarmament accord, in which it promised to shut down its main nuclear facility in exchange for economic and political concessions.

US officials have been saying since earlier this month that the financial obstacle had been overcome, but North Korea's statement Monday was the country's first confirmation that the money transfer was finished and the matter was fully wrapped up.

South Korea's decision to start shipping food aid was made "in consideration of the request of the public and international community on a humanitarian issue," Unification Minister Lee Jae-joung said at a press conference.

Lee said that South Korea notified North Korea earlier in the day regarding the decision.