North Korea has brought forward the possible date of a controversial "satellite" launch to as early as Sunday, regional governments say.
The secretive state will launch the satellite-bearing rocket between 7-14 February, the Japanese government has said, according to reports.
Pyongyang previously said the launch would take place between 8-25 February.
The planned launch has been condemned by world powers, which say it is a cover for testing a ballistic missile.
North Korea did not inform international organisations of any other changes in its plan and the rocket's expected flight path remains the same, said South Korea's defence ministry.
The South has warned the North that it will "pay a harsh price" if it goes ahead with its plan to launch the satellite.
Japan's defence 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 the North might do the launch ahead of 16 February, 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 6 January.
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.
The North insists its space programme 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.
North Korea's rocket launches
- May 2015: North Korea announces it has successfully tested a submarine-launched missile for the first time, but scepticism is then poured on the claim
- Dec 2012: North Korea launches three-stage rocket, says it successfully put a satellite into orbit; US defence officials confirm object in orbit
- Apr 2012: Three-stage rocket explodes just after take-off, falls into sea
- Apr 2009: Three-stage rocket launched; North Korea says it was a success, US says it failed and fell into the sea
- Jul 2006: North Korea test-fires a long-range Taepodong-2 missile; US said it failed shortly after take-off