North Korea told the United States it shut down its nuclear reactor, the State Department said Saturday, hours after a ship cruised into port loaded with oil promised in return for the country's pledge to disarm.

If confirmed by a U.N. inspection team headed to the Yongbyon reactor, the shutdown would be the North's first step in nearly five years toward de-nuclearization.

"We welcome this development and look forward to the verification and monitoring of this shutdown by the International Atomic Energy Agency team," State Department spokesman Sean McCormack said in a statement.

After tortuous negotiations and delays — during which the North argued its nuclear program was needed for self-defense — the reclusive regime said earlier this month that once it received the oil shipment, it would consider halting its reactor.

The 10-member team from the International Atomic Energy Agency arrived in the North Korean capital Saturday afternoon. Team chief Adel Tolba said they would stay in North Korea as long as needed to complete its work at the Yongbyon plutonium-producing reactor, about 120 kilometers (60 miles) northeast of Pyongyang.

"We are going directly to the nuclear site at Yongbyon," Tolba told broadcaster APTN outside the airport.

Footage showed dozens of cardboard boxes being loaded onto the back of two trucks. It was not immediately clear what they contained but Tolba earlier said he and his colleagues were bringing 1,000 kilograms (2,200 pounds) of equipment for use during their inspection trip

Top U.S. nuclear envoy Christopher Hill had earlier told reporters in the Japanese resort town of Hakone, south of Tokyo that he expected the North to shut down the reactor within days.

"I think it's a matter of today, tomorrow, maybe Monday," Hill said.

Hill also said he expected the North to submit a list of its nuclear facilities within months, as was agreed upon in February's round of disarmament talks.

"We expect the comprehensive list in a matter of several weeks, possibly several months," Hill said.

But he warned that the process was not likely to go smoothly. "I wish I could say we won't have any more problems, but experience tells me otherwise," Hill said.

After the IAEA team installs monitoring equipment, personnel will remain at Yongbyon to ensure the reactor remains shut down, said a diplomat familiar with North Korea's file at the IAEA.

"The IAEA plans to have a permanent presence there, with some experts remaining at the site continuously," said the diplomat, who requested anonymity due to the issue's sensitivity.

Saturday's delivery of 6,200 tons of heavy fuel oil was the first of 50,000 tons promised to the North in exchange for shutting down its reactor. Pyongyang will eventually get 1 million tons of oil and other financial and political concessions in a deal with the United States, China, Japan, South Korea and Russia.

The South Korean tanker No. 9 Han Chang arrived at the North's northeastern port of Sonbong, and the oil was being unloaded, a Unification Ministry official said. The South Korean official spoke on condition of anonymity because he was not authorized to talk to media.

The six-party agreement eased a standoff that began in October 2002, when the U.S. said North Korean officials had admitted having a secret uranium enrichment program. Washington said that violated a 1994 agreement for the North's disarmament, and a month later halted oil shipments under that deal.

The North reacted by expelling IAEA monitors, withdrawing from the Nuclear Nonproliferation Treaty and restarting the reactor.

North Korea has since occasionally shut down the reactor to remove fuel rods and extract plutonium. It is believed to have harvested enough for at least a dozen bombs.

The North set off an underground nuclear test explosion in October, leading to intensified international efforts to negotiate an end to its arms program.

The North was likely to term the shutdown simply a suspension of operations — which could be easily reversed.