The United States and South Korea will kick off a major navy drill next week, the US navy said on Friday, a fresh show of force against North Korea over its missile and nuclear tests.
The defence ministry said signs that North Korea was "preparing for another ballistic missile launch have consistently been detected since Sunday's test", referring to Pyongyang's sixth nuclear test.
The US may launch unilateral sanctions against North Korea, a US special envoy for the isolated state said on Sunday, two days after it carried out its fifth and biggest nuclear test in defiance of UN sanctions.
The Philippines said on Saturday it had seized a North Korean freighter that was covered by harsher United Nations' sanctions against Pyongyang over its nuclear programme.
The UN Security Council is considering imposing further sanctions on North Korea after it carried out a nuclear test, the British ambassador said Wednesday.