There are fears North Korea could stage a further missile test on Saturday the day it marks its founding anniversary.
Seoul says it is keeping close tabs on Pyongyang amid speculation it will launch a ballistic missile.
Last year, the north conducted its fifth atomic test on September 9 to mark its1948 establishment.
Last Sunday, in defiance of international sanctions, North Korean leader Kim Jong un ordered his country’s sixth atomic test.
President Donald Trump has vowed to stop the north by military means if necessary. Earlier the USS Ronald Reagan left its home port of Yokosuka near Tokyo. Washington has also called for a UN Security Council vote on Monday in a bid to impose tougher sanctions against the North.