North Korea is likely to conduct a long-range missile test in an "abrupt" manner, the Defense Ministry said Thursday. But the country has yet to declare a no-sail zone, a sign that no test launch will come in the very near future, it said.

"North Korea is likely to do it abruptly when they launch important provocative acts in the future," ministry spokesman Kim Min-seok said in a regular briefing.

Earlier in the day, Japan's Kyodo News agency reported that the North could launch a long-range missile in one week's time at the Dongchang-ri launch site in the country's northwest, citing satellite imagery analyses.

Kim neither confirmed nor denied the report, only saying that "Our military is keeping close tabs on the signs of North Korea's long-range missile launch."

The spokesman, however, said the North has yet to declare a no-sail zone, a notification necessary before the country conducts a long-range missile launch that affects other parts of the world.

"A no-sail zone is required internationally because North Korea's past long-range missiles or rockets flew to the east coast of the Philippines. But I have not heard so far on that," Kim noted.

If North Korea conducts a long-range missile launch, it would constitute a violation of United Nations Security Council resolutions and South Korea will jointly respond to it with the United States, he said.

The possibility of an imminent North Korean long-range missile launch is growing since the communist country's fourth nuclear test on Jan. 6.

In the past, the North has shown a pattern of conducting a long-range missile launch followed by a nuclear test as part of the country's push to develop intercontinental ballistic missiles.

In the latest long-range missile launch in December 2012, the North lifted off what it claimed was the Unha-3 carrier rocket to put the Kwangmyongsong-3 satellite into orbit.

South Korea and the international community, however, denounced the satellite launch as a cover for a test of what would be developed into an intercontinental ballistic missile, given the similarity in the technologies of space and missile launches.

Exactly how far the North has come in the development of an ICBM remains unknown, but the country is speculated to own long-range missiles that put part of the U.S. mainland as well as South Korea and Japan within its range of attacks.

With a success in the North's on-going efforts to miniaturize nuclear warheads, the communist country could have all three countries within range of nuclear attack.

Intelligence and military sources say North Korea technically stands ready to conduct a long-range missile test at any time since the country completed a major upgrade of the northwestern launch site last year.

"Since the completion of an extension on the launch site last year, the North is assessed to be able to conduct a sudden launch at any time," the sources said.

In the renovation work, the North extended the height of the missile launcher to 67 meters, making it possible for the country to launch a bigger and longer-ranged missile than the Unha-3, which flew over South Korea and Japan before landing in the waters near the Philippines.

But the country has covered the launch site since last year in an apparent attempt to keep any launch preparations secret from U.S. spy satellites, according the sources and satellite imagery.

Last year's renovation work made it possible for the North to prepare a missile launch in a few hours without the outside world knowing. (Yonhap)