U.N. nuclear inspectors verified the shut down of North Korea's reactor, confirming the most significant move to curb the North's atomic ambitions in years, but more remains to be done, the head of the IAEA said on Monday.

North Korea said over the weekend it had shut its Soviet-era Yongbyon nuclear reactor, which provides the secretive state with material for arms-grade plutonium, around the time it received the first shipment of 6,200 metric tons of oil provided by Seoul as part of an aid-for-disarmament deal.

"The reactor has been shut down," International Atomic Energy Agency director Mohamed ElBaradei told reporters in Bangkok. "We have verified the shutdown of the reactor."

The next step will be to verify that North Korea has shut other key facilities at Yongbyon, located about 100 km (60 miles north of Pyongyang -- which include a plant to make plutonium.

"It's a very important step that we are taking this week, but it's a long way to go," ElBaradei told reporters in Bangkok.He has said it will take IAEA personnel, who arrived in North Korea on Saturday, about a month to install seals and monitoring equipment to make sure Pyongyang keeps the reactor closed.

South Korea sent a second batch of 7,500 metric tons of oil to energy-starved North Korea on Monday, a Unification Ministry official said.

A provision of 50,000 metric tons of oil from the South is part of a February 13 deal reached among the two Koreas, the United States, Japan, Russia and China on first suspending the operation of the North's nuclear facilities and then disabling them.

The impoverished North will receive an additional 950,000 metric tons of oil, security assurances and be better able to conduct international trade if it completely scraps its nuclear arms program -- considered one of Asia's biggest security threats.