Seoul - South Korea said Thursday it had confirmed North Korea has started to reassemble its Yongbyon reactor, and expressed concern about the development, urging North Korea to stop turning back the "progress of denuclearization."

"We've confirmed North Korea has actually started the work to reassemble its reactor. And we urge North Korea to stop the work and not to further worsen the situation," South Korea's Foreign Ministry said in a statement released overnight.

The nuclear work started a week after the communist nation announced August 14 that it had stopped disabling the nuclear plant where it produced weapons-grade plutonium, complaining about the US failure to remove it from a list of state sponsors of terrorism.

"We are seriously concerned, because the work runs counter to the progress that has recently been made," South Korea said.

South Korea's ministry expressed concern the development will hurt the progress of the six-way talk with Russia, China, North Korea, Japan, the United States and South Korea, aimed at scrapping North Korea's nuclear weapon programme.

"We are closely at talk with the other participant countries at the six-way talks to decide how to react, and we keep our eyes on North Korea's attitudes. At the moment, however, we won't overreact," the ministry said.

Last October, North Korea promised to disable its Yongbyon nuclear reactor, and it blew up a cooling tower located next to the reactor in June. (dpa)