North Korea has moved up the window of its planned long-range rocket launch, South Korea's defence ministry said, adding that it believes the launch could come as soon as Sunday.
The launch, which the North says is an effort to send a satellite into orbit, would be in defiance of repeated warnings by outside governments that suspect it is a banned test of ballistic missile technology.
North Korea did not inform international organisations of any changes in its plan, and the rocket's expected flight path remains the same, said South Korean defence ministry spokesman Moon Sang Gyun, adding that the South believes the launch window now spans from February 7 until 14.
Satellite images taken this week of North Korea's Sohae rocket launch site showed apparent fuelling activity seen in the past shortly before a rocket launch, a US think-tank said, as US government officials signalled that the new launch could occur as early as Monday.
North Korea has told UN agencies it will launch a rocket carrying what it called an earth observation satellite sometime between February 8 and 25, triggering international opposition.
US government sources said on Friday that US intelligence agencies believed North Korea could be ready by Sunday evening, which will be Monday in Korea. Activity at the site was consistent with a launch in the timeframe given by Pyongyang, US officials said.
US Pacific Command said it was closely monitoring the situation and had many missile defence assets in the region that would provide "a robust defence".
"No one should doubt that US Pacific Command forces are prepared to protect the American homeland and defend our allies in South Korea and Japan," said Pacific Command spokesman US Navy Captain Cody Chiles.
Commercial satellite images from Wednesday and Thursday showed the arrival of tanker trucks at the launch pad, said Washington-based 38 North, a North Korea-monitoring project. It said the presence of the trucks 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 group said.
On Friday, US President Barack Obama spoke by telephone with President Xi Jinping of China, North Korea's main ally and neighbour, and agreed that a North Korean launch would represent a "provocative and destabilising action" as the White House said.
Obama and Xi also said they would coordinate efforts to respond to North Korea's nuclear test last month and said they would not accept North Korea as a nuclear weapon state.
"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.
Washington and Beijing have appeared divided over how to respond to North Korea, with the US urging tougher sanctions and China stressing the need for dialogue.
Earlier on Friday, Xi told South Korea's president that China was dedicated to maintaining peace and stability on the Korean peninsula.
North Korea says it has a sovereign right to pursue a space programme. But it is barred under UN Security Council resolutions from using ballistic missile technology.
Coming so soon after North Korea's fourth nuclear test on January 6, a rocket launch would raise concern that it plans to fit nuclear warheads on its missiles.