North Korea has fired a long-range rocket which critics see as cover for a banned missile test.
On Friday, US intelligence agencies announced that North Korea could possibly launch the rocket on Sunday since satellite images taken over North Korea's Sohae rocket launch site showed apparent fueling activity.
Activity at the site was consistent with a launch in the time frame given by Pyongyang, US officials said.
North Korea has told UN agencies it will launch a rocket carrying what it called an earth observation satellite some time between Feb. 8 and Feb. 25, triggering international opposition from governments that see it as a long-range missile test.
"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.
See photos below of Japan's military alert about the rocket launch:
On Friday, US President Barack Obama spoke by telephone with President Xi Jinping of China, North Korea's main ally and neighbor, and agreed that a North Korean launch would represent a "provocative and destabilizing action," 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 emphasized 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 United States 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.
Space launch or ballistic missile test?
The 38 North report said activity could also be seen around a building at the launch site used in the past to receive and assemble rocket stages.
It said the imagery showed vehicles including one or two buses and a crane, a level of activity similar to that seen before the previous launch in 2012.
The group said the images indicated no significant changes at the launch pad itself, where work platforms on the gantry towers remained folded forward. It said coverings obscured whether a space-launch vehicle was present on the pad.
North Korea says it has a sovereign right to pursue a space program. But it is barred under U.N. Security Council resolutions from using ballistic missile technology.
Coming so soon after North Korea's fourth nuclear test on Jan. 6, a rocket launch would raise concern that it plans to fit nuclear warheads on its missiles, giving it the capability to strike South Korea, Japan and possibly the US West Coast.
US Pacific Command said it was closely monitoring the situation and had many missile defense assets in the region that would provide "a robust defense."
"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 Air Force Captain Cody Chiles.
US Pacific Command said it had Aegis ballistic missile defense systems, Terminal High Altitude Area Defense batteries and the Sea-Based X-Band Radar in the region, which would work with Japanese and South Korean militaries to detect the launch.
The USS Benfold, a guided-missile destroyer, arrived on Friday in Otaru, Japan, one of five US destroyers equipped with Aegis ballistic missile defense systems that are girding for the potential launch, according to several sources.
Harris also told reporters after his speech at the Center for Strategic and International Studies in Washington that it made sense to put a mobile missile-defense system known as the Terminal High Altitude Area Defense (THAAD) in South Korea.
The THAAD system's mobility and strategic battery-unit placement is designed to counter threats around the globe. In April 2013, the Pentagon deployed a THAAD battery to Guam in order to deter North Korean provocations and further defend the Pacific region.
The THAAD missile does not carry a warhead, instead designed to use pure kinetic energy to deliver "hit-to-kill" lethality to ballistic missiles inside or outside of the atmosphere.
The ships are spread out around the region to protect installations in Japan, Guam and South Korea, and to track the first and second stages rocket boosters as they fall to earth after the launch.
(Reporting by David Brunnstrom; Additional reporting by Andrea Shalal, Ayesha Rascoe and Mark Hosenball in Washington and Ben Blanchard in Beijing; Editing by David Alexander and James Dalgleish)
See photos of North Korea's nuclear test below: