A top Japanese official is calling for U.N. economic sanctions against North Korea if Pyongyang tests a nuclear device.

Shinzo Abe, acting secretary general of Japan's ruling Liberal Democratic Party, said Sunday that it would be "unthinkable" for Tokyo to do nothing should North Korea conduct such a test.

U.S. officials have warned that North Korea may be preparing for a nuclear test, citing satellite photos they say show suspicious activity near the coastal town of Kilju.

Interviewed on the television program Fox News Sunday,  U.S. National Security Adviser Stephen Hadley said a nuclear test would be an act of defiance, and prompt Washington and its allies to discuss new steps to punish the North Korean government.

North Korea has warned that it will consider any sanctions imposed against it a declaration of war.

Some information for this report provided by AFP and AP.