North Korea conducted nuclear tests in 2006 and 2009.
Tuesday’s test was carried out using a “miniaturized and lighter nuclear device with greater explosive force than previously,” the news agency said.
U.S. intelligence officials assessed that the explosion yield was several kilotons.
Mr. Obama said North Korea’s nuclear-weapons and ballistic-missile programs constitute threats to U.S. national security and to international peace. He also spoke on the phone Tuesday with South Korean President Lee Myung-bak.