Military chiefs of South Korea and the United States on Wednesday pledged close cooperation in responding to the North's planned rocket launch and a potential nuclear test, a Seoul official said.
The chairman of South Korea's Joint Chiefs of Staff (JCS), Jung Seung-Jo, had a telephone discussion with his US counterpart Martin Dempsey and the top US Pacific Commander Samuel Locklear, a JCS spokesman told AFP.
"The three assessed the situation on the rocket launch plan and agreed to closely cooperate in case of another provocation like the North's potential nuclear test," he said.
The impoverished but nuclear-armed North is set to launch a rocket between 12 and 16 April which it says is to put a satellite into orbit to mark the centenary of the birth of its late founding president Kim Il-Sung.
Countries including the US, the South and Japan see it as a disguised long-range ballistic missile test banned under UN resolutions. The US has suspended planned food aid to the North that had been offered in return for a freeze of some nuclear and missile activities.
Pyongyang however said Wednesday that the fuelling of the rocket was underway in defiance of international condemnation, reiterating its claim that the launch is a peaceful space project.
The South's military has stepped up its alert ahead of the controversial launch by sending warships and Aegis destroyers to the Yellow Sea off the west coast to detect the trajectory of the rocket, Yonhap news agency said.
"The missile-detection systems of the US and the South's military are currently under operation," Yonhap news agency quoted a military source as saying.
The South Korean-US combined forces in Seoul have raised a five-stage surveillance system on the North's military movements by one notch to the second-highest, it added. A spokesman for the US forces declined to comment.
A Seoul official has said the North appears to be preparing for a third nuclear test in the northeastern town of Punggye-ri, where it carried out two previous tests in 2006 and 2009.
Recent satellite images indicate construction of a new underground tunnel for staging another nuclear test is almost complete, the official told AFP on Sunday on condition of anonymity.
The North, believed to have enough plutonium for six to eight bombs, tested atomic weapons in October 2006 and May 2009. Both were held one to three months after missile tests.