There is concern in Japan and South Korea that North Korea might be preparing to launch a ballistic missile.
Officials in both Japan and South Korea say that since Tuesday, they have detected signs of activity around a suspected launch site for North Korea's Rodong missiles.
Media reports in both countries on Thursday said intelligence satellites and radio transmissions indicate an increase of personnel and equipment at the site.
However, it is not clear if North Korea is preparing to actually launch a missile or is conducting an exercise simulating a launch.
In Seoul, Deputy Unification Minister Rhee Bong-jo told reporters that his government is trying to confirm what is taking place.
Mr. Rhee says it is likely the missile units are conducting a routine annual drill but the possibility of an actual launch cannot be ruled out.
A spokesman at Japan's Foreign Ministry says the situation is "not at a stage where a formal statement is warranted" - a reference to media reports that there is no indication liquid fuel has been loaded into any missiles.
News reports say Japan has dispatched an Aegis-equipped destroyer and surveillance aircraft to the Sea of Japan to monitor the situation.
In recent years North Korea has carried out similar drills that were apparently simulated launches. Last year, it also test fired several short-range anti-ship missiles.
The Rodong missiles can strike much of Japan. North Korea has a larger missile, which it fired over Japan in 1998, unnerving officials in Tokyo.
North Korea's leader, Kim Jong Il, pledged to Japanese Prime Minister Junichiro Koizumi two years ago that Pyongyang would freeze missiles launches "in and after 2003."
Earlier this month, South Korean reports that a large explosion had taken place in North Korea sparked fears that Pyongyang had been preparing a missile launch or had tested a nuclear device. Later, however, South Korean officials said their intelligence analysts appeared to have been wrong, and no major blast had occurred.
The reports of activity near the missile site come amid an impasse between North Korea and five other nations over holding a fourth round of talks about the communist state's nuclear weapons programs. Talks involving both Koreas, the United States, China, Japan and Russia had been planned for late this month in Beijing.