The planned launch - said to be for an earth observation satellite - would constitute a violation of UN sanctions
North Korea is ready to launch what it has called an earth observation satellite as early as tomorrow.
It had announced it would launch a rocket carrying it sometime between 8 and 25 February, around the time of the birthday of late leader Kim Jong-Il, the father of Kim Jong-Un.
However, South Korea's Defence Ministry said on Saturday Pyongyang had amended its notice to bring forward the time frame for the launch to between 7 and 14 February.
Japanese media, including Kyodo, Jiji Press and NHK news agencies, reported the Japanese government as saying North Korea had informed the International Maritime Organisation of the new schedule.
This was disputed by Seoul which said Pyongyang had not informed international bodies of any changes.
Satellite images analysed by US researchers apparently show the arrival of fuel tankers at North Korea's Sohae rocket site, prompting speculation that an early launch was on the cards.
The launch would constitute a violation of UN sanctions as similar technology can be used for ballistic missiles.
A report by Washington-based 38 North, a North Korea-monitoring project, said the presence of the fuel trucks at the launch pad "likely indicated the filling of tanks within bunkers at the site rather than a rocket itself".
"In the past, such activity has occurred one to two weeks prior to a launch event and would be consistent with North Korea's announced launch window," the report added.
A US government source said US intelligence agencies believed North Korea could be ready by the US Super Bowl kick-off on Sunday, which will be Monday, Korea time.
On Friday, US President Barack Obama spoke by telephone with Chinese President Xi Jinping of China, North Korea's main ally, and agreed that the secretive nation's planned launch would represent a "provocative and destabilising action," the White House said.
The two leaders said they would also co-ordinate efforts to respond to North Korea's purported hydrogen bomb test last month.
"The leaders emphasised the importance of a strong and united international response to North Korea's provocations, including through an impactful UN Security Council Resolution," the White House said.