SEOUL, South Korea --The Pentagon is calling North Korea's nuclear test "yet another flagrant violation" of U.N. Security Council resolutions as well as a "serious provocation."
Pentagon press secretary Peter Cook is traveling Friday in Norway with U.S. Defense Secretary Ash Carter. In a statement, Cook says Carter has been briefed on reports of seismic activity near a North Korea nuclear site. Cook says Carter will remain in close contact with America's South Korean allies as well as others friends and allies in the region.
Cook says a nuclear test would pose "a significant threat to the peace and security of the Korean peninsula and the stability of the Asia-Pacific region."
North Korea confirmed Friday it had tested a nuclear warhead designed to be mounted on ballistic missiles.
Japan's Chief Cabinet Secretary Yoshihide Suga called North Korea an "outlaw nation in the neighborhood" following Pyongyang's fifth nuclear test.
Suga says Japan will consider stepping up its own sanctions in addition to what it already has in place, along with those imposed by the U.N. Security Council.
Japan currently bans entry of North Korean nationals and re-entry of senior members of North Korean permanent residents' association in Japan. It also has a ban on port entry of all North Korean vessels.
Meantime, U.S. Secretary of State John Kerry says he is deeply concerned about North Korea.
In Geneva for meetings about Syria, Kerry says he spoke Friday with the foreign ministers of Japan and South Korea. He says "everybody shares concerns" about the situation on the Korean peninsula right now.
Kerry says the U.S. is still trying to determine precisely what happened. He didn't refer to Friday's event as a nuclear test.
He spoke as he started a day of Syria negotiations with Russian Foreign Minister Sergey Lavrov.
Lavrov says he will talk to Japanese Foreign Minister Fumio Kishida soon. He says U.N. Security Council resolutions on North Korea must be respected "and we must send this message very strongly."
Japan's Chief Cabinet Secretary Yoshihide Suga says Prime Minister Shinzo Abe and President Barack Obama held telephone talks and agreed to cooperate in seeking an emergency U.N. Security Council meeting to discuss a possibility of effectively imposing sanctions on North Korea over the nuclear test.
The Security Council in March imposed the toughest sanctions on North Korea in two decades, reflecting growing anger at Pyongyang's nuclear test and rocket launch earlier this year in defiance of a ban on all nuclear-related activity.