A Japanese daily on Sunday said North Korea has reactivated its nuclear reactors in Yongbyon ahead of the resumption of the six-way nuclear talks.

The Asahi Shimbun quoted a senior U.S. official and other sources close to the talks that a U.S. spy satellite confirmed steam coming from a boiler connected to the reactors.

Photos also showed gravel roads being laid near by a 50-megawatt reactor under construction.

The U.S. official said the pictures indicate that Pyongyang has recharged nuclear fuel rods to reactivate the facility.

He said the reactivation and reactor construction in Yongbyon are on the agenda for the upcoming nuclear talks in Beijing, warning that the North is expanding its nuclear program though apparently willing to give up the reactors.