S. Korea, US maintaining high-level surveillance since early March

South Korea and the United States have been maintaining a high level of vigilance against North Korea since early March, the presidential spokesman said Thursday, denying reports the surveillance level was raised in response to Pyongyang's moves to fire a ballistic missile.

The alertness, dubbed "Watchcon," was upgraded by one step to Level 2 from Level 3 on March 5 when North Korea's military said it will void the Armistice Agreement that halted the 1950-53 Korean War, presidential spokesman Yoon Chang-jung told reporters.

Local media had reported Wednesday that Watchcon was raised recently to keep close watch over the North's moves to launch a medium-range ballistic missile, known as Musudan, this week ahead of the April 15 birthday of late North Korean founder Kim Il-sung.

"The Korea-U.S. intelligence surveillance assets have been bolstered since the (North's) Supreme Command of the Korean People's Army issued a statement," Yoon said, referring to Pyongyang's threat to void the cease-fire. "It is not true that it was raised yesterday."

South Korean officials have said the North could launch the Musudan missile at any time. The launch, if carried out, would mark the first test firing of the longer-range missile believed to be capable of flying as far away as the U.S. territory of Guam in the Pacific.

Experts said the North appears to be trying to use the launch to demonstrate its capabilities to deliver nuclear warheads on longer-range missiles. In December, the North also successfully sent a satellite into orbit on a long-range rocket, a move widely condemned as a banned test of missile technology.

North Korea has long used missile or rocket launches not only to hone its missile capabilities, but also as a way of expressing anger at the outside world. Pyongyang has been upset this time over new U.N. sanctions for its February nuclear test and U.S.-involved annual military drills in the South.

About 28,500 American troops are stationed in South Korea to deter threats from the North, a legacy of the Korean War, which ended without a peace treaty, leaving the divided peninsula still in a state of conflict.