According to Japan’s government, North Korea will launch the satellite-bearing rocket between February 7 and 14.
Pyongyang previously said the satellite launch would take place between February 8 and 25.
The planned launch has been condemned by world powers, which say it is a cover for testing a ballistic missile.
South Korea has warned the North that it will “pay a harsh price” if it goes ahead with its plan to launch the satellite.
Japan’s defense minister said he had issued an order to shoot down any missile that threatened to fall on Japanese territory.
South Korean analysts have speculated that North Korea might do the launch ahead of February 16, the birthday of the late North Korean dictator Kim Jong-il.
North Korea has already provoked international criticism this year with a fourth nuclear bomb test on January 6.
A launch in the coming weeks would constitute another major violation of UN Security Council resolutions banning the state from carrying out any nuclear or ballistic missile tests.
North Korea insists its space program is purely scientific in nature, but the US, South Korea and even ally China say the rocket launches are aimed at developing an inter-continental ballistic missile capable of striking the US.