North Korea said it has restarted its main Yongbyon nuclear complex Tuesday, and added that it is ready to respond to "U.S. hostility" with nuclear weapons "at any time."

In a statement attributed to the director of the Atomic Energy Institute, the secretive nation's state-controlled Korea Central News Agency (KCNA) reported that North Korea's "access to nuclear weapons is an outcome of the US hostile policy towards it."

A file photo taken by Chinese state media shows the cooling tower at the Yongbyon complex prior to it being destroyed, December 2007.

The statement had been given in direct response to western reports of satellite data showing fresh activity at Yongbyon, the KCNA said.

Yongbyon was shut down in 2007, after North Korea agreed to disarm in exchange for fuel aid. However, Pyongyang said it would restart the plant in 2013.

In April, think tank the Institute For Science and International Security (ISIS) said patterns of melting snow in satellite images taken of the area around Yongbyong showed increased activity at the plant.