North Korea informed a U.N.maritime agency Saturday that it will move up its plan to launch a long-range rocket to between Feb. 7 and 14, the Seoul government said.

South Korea said it is already on full alert for the launch that could take place as early as Sunday, noting that the country's military maintains an immediate response posture.

Earlier, Pyongyang informed the International Maritime Organization (IMO) and other United Nations agencies of its plan that the launch would take place between Feb. 8 and 25.   

"North Korea informed the IMO today that it will amend the date of its planned missile launch to between Feb. 7 and 14," said a South Korean government official on the condition of anonymity.

Seoul says the trajectory of the missile and other details are the same - just the launch window has been adjusted.

The Seoul government held an emergency meeting presided over by Foreign Minister Yun Byung-se to counter the possible launch.

"(The military) stays on readiness posture to respond whenever North Korea fires a missile," said a defense ministry official on the condition of the anonymity.

Experts earlier expected that the North would likely launch the rocket around Feb. 16, the birthday of the former North Korean leader Kim Jong-il, who died due to heart failure in 2011

"Weather in the North Korea Dongchang-ri launch site area is expected to be good between Feb. 7 and Feb. 10," said the official, forecasting the launch could take place as early as on Sunday.

The Seoul government forecasts that Pyongyang has completed preparation, including the fueling of the rocket at the launch pad on its rocket site.

"The Seoul government believes that (North Korea) completed its launch preparation, such as fueling the rocket after erecting it on a launch pad after considering various circumstances," said another defense ministry official.

The Korea Meteorological Administration, Seoul's weather agency, said that rain or snow was forecast in the launch site region on Monday afternoon.

Meanwhile, it is expected to be sunny on Sunday, prompting experts to speculate that the North may pull off the launch earlier.

"It is safer to pull off the launch as soon as possible considering the instability of the liquid fuel," said a missile expert, citing that North already loaded fuel into a rocket.

Pyongyang notified the IMO of its change of launch window Saturday morning but did not send similar notices to other U.N agencies such as the International Civil Aviation Organization and the International Telecommunication Union.

Earlier in the day, the Japanese government confirmed Pyongyang's plan to move forward the launch period, according to Japan's Kyodo News Agency. 

"We are in the process of confirming since there is so much information," Gen Nakatani, Japan's defense minister, was quoted by Kyodo News as saying.

Ahead of the notification, the U.S. Pacific Command called on North Korea to refrain from "irresponsible provocations," responding to its move to fire a long-range rocket.

"No one should doubt that U.S. Pacific Command forces, including U.S. forces in Korea, are prepared to protect the American homeland and defend our allies in South Korea and Japan,"

Hawaii-based command spokesman U.S. Navy Capt. Cody Chiles told Yonhap News Agency.

The North looks set to launch what the outside world views as an intercontinental ballistic missile.

"We are aware of North Korea's announcement of their rocket launch and are closely monitoring the situation. We continue to call on North Korea to refrain from irresponsible provocations," Chiles said.

He added that the U.S. operates many platforms in the Pacific including Aegis ships, the THAAD advanced missile defense system, sea-based X-band radar, and other assets.

Earlier in the day, Defense Minister Han Min-koo visited an Aegis-equipped destroyer tasked with immediately detecting North Korea's long-range rocket in case of a launch, his ministry said. 

Aboard the 7600-ton KDX-III destroyer Seoae Ryu Seong-ryong, Han instructed the troops to maintain full combat posture with North Korea poised to launch what it claims to be a space vehicle.

Also, top military officers of both South Korea and the U.S made a joint tour of the Joint Security Area inside the truce village of Panmunjom.

Gen. Lee Sun-jin, chairman of South Korea's Joint Chiefs of Staff (JCS), and U.S. Forces Korea commander Gen. Curtis Scaparrotti called for full combined combat readiness, according to the JCS.

North Korea argues the launch is a space vehicle, a claim refuted by South Korea which contends that long-range rockets and intercontinental ballistic missiles are basically the same.

The North has repeatedly pledged to boost its nuclear capabilities, viewing its nuclear program as a powerful deterrent against what it claims is Washington's hostile policy toward it.

U.N. sanctions prohibit North Korea from any launch using ballistic missile technology. (Yonhap)