There is growing concern in Northeast Asia that North Korea is about to conduct another provocative missile launch, possibly firing a number of rockets simultaneously.
South Korean and U.S. forces in the country have gone to a higher reconnaissance posture, just one notch below that of wartime.
The status change from Watch Condition (Watchcon) 3 to 2 comes amid indications of the presence of missiles on mobile launchers in North Korea.
Watchcon 2 denotes indications of a "vital threat." Domestic South Korean media say the change went into effect Wednesday.
Foreign Minister Yun Byung-se, appearing before a legislative committee in Seoul, characterizes rival North Korea as ready to launch a medium-range Musudan ballistic missile.
Yun says intelligence obtained by South Korea and the United States has ascertained that “the possibility of a missile launch by North Korea is very high” and could occur “at any time from now.”
Yun describes the Musudan, which Pyongyang has never tested, as having a range of about 3,500 kilometers and “it is up to North Korea how far it would fly.”
South Korea's semi-official news agency, Yonhap, says that additional mobile launchers - possibly for other types of missiles -- have been spotted in South Hamgyong province on the east coast.
A top U.S. military officer, Admiral Samuel Locklear, earlier confirmed before the Senate Armed Services Committee in Washington that at least one medium-range ballistic missile had been moved to the east coast of North Korea.
Locklear, who heads the Pacific Command, says the U.S. military is able to intercept a ballistic missile fired by North Korea that would threaten U.S. bases or any of America's allies in the Pacific, including South Korea and Japan.
The committee's chairman, Senator Carl Levin, asked Locklear for details about a new joint counter-provocation plan with South Korea.
It outlines types of responses to limited military action by North Korea.
The admiral says he is hesitant to provide specifics during open testimony.
"I am supportive of the plan and I think it's a good one. And we will continue to revise it as time goes on," he said.
Senator Levin asked "But are you satisfied that we would be ready if there were such a limited military action from North Korea?"
"I am satisfied that we're ready today, yes," Locklear responded.
Related video report by Luis Ramirez
Mood in Seoul
Despite the increasing tension on the peninsula, the mood remains calm in the South Korean capital, Seoul, and the metropolitan area where more than 20 million people reside - nearly half the country's population.
The U.S. State Department says it is not advising its citizens to consider leaving nor is there any reason to avoid visiting the country.
North Korea on Tuesday suggested that foreigners in the South should depart because all-out war could break out at any time.
South Korea's presidential office says Pyongyang's attempt to create fear among foreigners will fail and it is merely psychological warfare.
The unusual advisory to expatriates in the South is the latest in a series of statements made by the North that have prompted international concern and condemnation.
After recent ballistic missile and nuclear tests, Pyongyang renounced the 1953 ceasefire agreement, vowed to conduct a preemptive nuclear strike on U.S. territory and declared a state of war between the North and South.
Despite all of this, military and intelligence sources say there are no indications North Korea has mobilized its army, which is one of the world's largest.
Meanwhile, Pyongyang is keeping 50,000 workers away from the only joint venture between the two Koreas, a factory complex at Kaesong.
Seoul Wednesday blamed the Reconnaissance General Bureau of the North Korean Army for a cyber attack last month that crippled 32,000 computers and servers at some South Korean broadcasters and banks.
South Korea's finance ministry is vowing to swiftly take stabilizing measures, should inter-Korean tension trigger strong volatility in stock or currency trading.
North Korean children hold up red scarves to be tied around their necks during an induction ceremony into the Korean Children's Union held at a stadium in Pyongyang, April 12, 2013.
Two military officers admire displays at a flower show featuring thousands of Kimilsungia flowers, named after the late North Korean leader Kim Il Sung, Pyongyang, April 12, 2013.
South Korean soldiers stand guard at an observation post near the demilitarized zone (DMZ) which separates the two Koreas in Paju, north of Seoul April 11, 2013.
Female North Korean soldiers patrol along the banks of Yalu River, near the North Korean town of Sinuiju, opposite the Chinese border city of Dandong, April 11, 2013.
A North Korean man blocks his face with his hand from being photographed as he and other residents take a ferry in Yalu River, near the North Korean town of Sinuiju, opposite the Chinese border city of Dandong, April 11, 2013.
People take part in an oath-taking before the statues of late North Korean leaders Kim Il-sung and Kim Jong-il on Mansudae Hill in Pyongyang, April 10, 2013. (KCNA)
Anti-North Korean protesters release balloons with peace messages on the Grand Unification Bridge leading to the North near the demilitarized zone separating the two Koreas, in Paju, north of Seoul, April 10, 2013.
South Koreans arrive with their belongings from North Korea's Kaesong at the customs, immigration and quarantine office near the border village of Panmunjom, in Paju, north of Seoul, South Korea, April 9, 2013.
Visitors look at the industrial complex in Kaesong, North Korea, through binoculars at Dora Observation Post in the demilitarized zone (DMZ) near the border village of Panmunjom, in Paju, South Korea, April 9, 2013.
A South Korean military vehicle passes by gates leading to the North Korean city of Kaesong at the customs, immigration and quarantine office near the border village of Panmunjom, April 8, 2013.
An elementary school teacher orders her students to leave as they watch South Korean housewives denounce annual South Korean-U.S. military exercises, near the U.S. Embassy in Seoul, April 8, 2013.
South Korean army soldiers patrol along a barbed-wire fence near the border village of the Panmunjom, in Paju, South Korea, April 8, 2013.
North Korean military dogs run to a target with a portrait of South Korean Defence Minister Kim Kwan-jin during a military drill, April 6, 2013. (KCNA)