North Korea has restarted its nuclear fuel reprocessing plant in an attempt to produce more weapons-grade plutonium, media reports said on Wednesday.
Steam was recently seen coming from a plant at Yongbyon, north of Pyongyang, in a sign it is being reactivated, South Korea's Chosun Ilbo newspaper said in a report that came two days after the North carried out a nuclear test.
"US spy satellites recently spotted various signs of the once frozen reprocessing facility being reactivated, such as water vapour coming from it," an unidentified official told the paper.
Yonhap news agency carried a similar report.
The North announced on April 14 it was quitting a six-nation nuclear disarmament agreement and would reopen the Yongbyon plant.
It had closed the reactor and other facilities in July 2007 as part of a disarmament deal and was working to disable them.
North Korea said its move was prompted by the UN Security Council's decision to censure its April 5 rocket launch and to tighten sanctions.
Chosun said the apparent restarting of the facility came earlier than predicted, since experts had expected it to take two to four months.
It said that if the North reprocesses all the 8,000 spent reactor fuel rods at Yongbyon, it could obtain another six to eight kilograms (13-18 pounds) of plutonium, enough to make one nuclear weapon.
The North said last June it had a stockpile of 31 kilograms of plutonium.
A diplomatic source quoted by Yonhap said the activity began soon after the North's declaration that it was quitting the disarmament talks.
"In mid-April, the door to the storage facilities which contain spent fuel rods was opened many times," the source said.
"Since late April steam has been coming from the plant, which produces water vapour for reprocessing (spent fuel rods).
"In the middle of this month, vehicles carrying chemicals were seen moving. Some kind of work is under way."
South Korean military authorities refused to comment on the Chosun report. The National Intelligence Service was not immediately available for comment.