The U.N. atomic watchdog chief on Monday called North Korea's drive to restart a nuclear site "troubling" and said his team could not detect whether the regime planned a new nuclear test due to a lack of access.

Yukiya Amano, director general of the International Atomic Energy Agency, said the watchdog had observed Pyongyang's efforts to rebuild parts of its Yongbyon nuclear site amid a showdown with South Korea and the United States.

"That is very troubling because they are against United Nations Security Council resolutions," Amano told reporters at a conference on nuclear policy held by the Carnegie Endowment for International Peace in Washington.

Amano said the IAEA had little knowledge of North Korean activities and relied largely on satellites since Pyongyang kicked out the agency's staff in 2009.

"Our knowledge on the activities of North Korea is rather limited. I cannot speculate when and if North Korea will conduct another nuclear test," Amano said.

South Korea said earlier that North Korea appeared to be preparing its fourth nuclear test, as well as a missile launch. But South Korean officials later backtracked, saying activities at the North's Punggye-ri test site were routine.

Tensions have soared in recent weeks over North Korea, which has responded furiously to United Nations condemnation of its nuclear test in February and its launch of a long-range rocket that put a small satellite into orbit.

North Korea said last week that it would restart all facilities at Yongbyon, where it had knocked down a cooling tower in 2008 to show it was serious about U.S.-supported talks on denuclearization. (AFP)