Japan's Kyodo News Agency says North Korea has resumed construction on two nuclear reactors that had remained frozen under a 1994 deal with the United States.

Kyodo said Wednesday a Standford University professor reported on the construction's resumption to US Secretary of State Condoleezza Rice last week after returning from a Pyongyang visit late last month.

The professor quoted the North's officials as saying construction started again on a 50-thousand kilowatt reactor and a 200-thousand one at the Yongbyon complex.

But a US official said Washington cannot verify the authenticity of the North's claim.

The graphite-moderated reactors are known to take several years to complete.