US envoy Christopher Hill said the closure was only the first step of a deal agreed in February but since subjected to numerous delays.
Pyongyang has now begun to receive the heavy fuel oil shipments it agreed to take in exchange for the closure.
The UN nuclear team is making its first trip to the North in five years.
Mr Hill, the chief US nuclear envoy, said in Tokyo: "We understood [that Yongbyon would shut down] this weekend, so I don't know whether it's Saturday, Sunday or Monday. I do know it's very soon."
But he added: "I don't want people to think this shutdown is the biggest and only event. It's just the first step."
Mr Hill said he expected a full list of the North's nuclear facilities within months - as agreed in the February deal.
Funds wrangle
The 10-member UN team will verify the shutdown at Yongbyon, 90km (56 miles) north of Pyongyang.
The International Atomic Energy Agency members are to decommission and seal equipment at the reactor and plutonium reprocessing plant.
It may take the inspection team up to three weeks to complete the task, says the BBC's Kevin Kim in Seoul.
In nuclear talks held in Beijing in February, North Korea was promised heavy fuel oil in exchange for the initial nuclear shutdown.
The deal was delayed amid a wrangle over North Korean funds frozen in a Macau bank account.
But the first shipment of oil has now arrived in North Korea.
The ship - No 9 Han Chang - docked at the North Korean port of Sonbong at 0920 on Saturday (0020 GMT), loaded with 6,200 tonnes of heavy fuel oil.
Talks involving South and North Korea, Russia, Japan, the United States and China are set to resume in Beijing on Wednesday to map out the next stage of disabling the North's nuclear programme.
North Korea tested an atomic bomb for the first time last year, and has repeatedly said it needs nuclear weapons to fend off a US attack.
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