"We visited all the places which we are planning to visit and co-operation was excellent," Olli Heinonen, the IAEA deputy director told reporters after returning to Pyongyang on Friday.
"The reason that the N. Koreans are so poor and hungry is because their communist government is completely inept"
Tom Dougherty, Atlanta, USA
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He said the reactor, located about 100km north of Pyongyang, remained operational and more discussions with North Korean officials would take place.
"It's not yet the point of shutdown so that is still to come," he said.
The visit was the first by an IAEA team since inspectors were expelled from the country in 2002.
The 5-megawatt reactor at Yongbyon, believed capable of producing enough plutonium for one atomic bomb a year, is at the centre of efforts to halt North Korea's nuclear programme.
Other facilities Heinonen's team saw at Yongbyon included an unfinished 50-megawatt reactor, the fuel fabrication plant and reprocessing plant.
Earlier in Washington Condoleezza Rice, the US secretary of state, said she was hopeful of a swift shutdown of the Yongbyon plant.
"We hope for now rapid progress given the beginning, we believe, of the North Korean efforts to meet their initial action obligations,'' Rice said.