Japan is closely monitoring a rocket launch by the Democratic People's Republic of Korea (DPRK) as its launch window has opened Monday morning, though the DPRK suggested it may postpone the launch.

Prime Minister Yoshihiko Noda said in the morning that Japan will keep vigilant against the planned launch.

Noda and his cabinet ministers, including Foreign Minister Koichiro Gemba and Defense Minister Satoshi Morimoto entered their offices before 7 a.m., the start of the daily five-hour launch window of the launch, reported Kyodo News Agency.

Morimoto said Japan will not downgrade the alert level before any postponement has been confirmed by the government or the DPRK officially announces the delay.

The DPRK said Saturday that the scheduled time for the satellite launch will be readjusted for some reasons. The launch was planned to take place between Dec. 10 and 22 from 7 a.m. to noon.

The DPRK said the rocket is to put a satellite into orbit. However, some countries believed the launch is DPRK's test for long-range ballistic missile.

To deal with the launch, Japan has completed deployment of its Self-Defense Forces (SDF) Saturday to prevent any wreckage of the satellite from falling into Japanese territory.

SDF's warships and the Patriot Advanced Capability-3 (PAC-3) missile interceptors have been deployed in Japan's Okinawa. Three units of the PAC-3 interceptors were also deployed in central Tokyo and places close to the capital.

Earlier Monday, two U.S. spy planes took off from Kadena Air Base in Okinawa to monitor the DPRK's rocket, Kyodo reported.

If the United States and Japan detect the rocket has been launched, the Japanese government will immediately notify local government and mass media and to decide on the next steps to cope with new development.