The US has seen increased activity around a North Korea site suggesting movement of components and propellant to be able to conduct a space launch in the near future, a US official told Reuters on Thursday.
"Our concern though is that they do a space-launch but really it's the same technology to develop ICBMs," the official said, referring to inter-continental ballistic missiles.
The official added such a launch could happen within a couple of weeks.
On Wednesday, Japan's Kyodo news agency cited an unnamed Japanese official saying that North Korea may be preparing to launch a long-range missile as soon as in a week.
The official cited signs of possible preparations for a missile launch based on analysis of satellite imagery of the North's Tongchang-ri missile-test site on its west coast.
The report came as UN Security Council members were discussing fresh sanctions against North Korea after it conducted its fourth nuclear test on January 6.
North Korea is already under sanctions for its nuclear and missile programs.
North Korea last conducted a long-range rocket launch in late 2012, successfully putting an object into orbit in what experts believed to be part of its effort to build an intercontinental ballistic missile.
The North is also seen to be working to miniaturize a nuclear warhead to mount on a missile, but many experts say it is some time away from perfecting the technology.
The Kyodo report gave no other details about the satellite-imagery analyses.
Meanwhile, the US Senate Foreign Relations Committee approved legislation to broaden sanctions on North Korea over its nuclear program, human rights record and cyber activities.
The committee passed the measure by unanimous voice vote and members said they expected it would be approved by the full Senate and, eventually, signed into law by President Barack Obama.
On Wednesday in Beijing, US Secretary of State John Kerry and Chinese Foreign Minister Wang Yi agreed on the need for a significant new UN Security resolution against the North, but there were few signs of concrete progress.
US Navy Admiral Harry Harris, commander of US Pacific Command, said on Wednesday before the Kyodo report was published that North Korea's actions underscored the importance of strengthening an alliance among Japan, South Korea, and the US.
He said he supported reviewing the possibility of converting a US Aegis missile-defense test site in Hawaii into a combat-ready facility to bolster US defenses against ballistic-missile attacks, an initiative first reported by Reuters last week.
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.
That deployment decision must be made jointly by the US and South Korea, Harris said.
North Korea said on January 6 that it had exploded a hydrogen bomb, although the US, other governments, and experts voiced skepticism that it had made such a technological advance.
(Reporting by Ju-min Park and Jack Kim; Editing by Bernard Orr and Richard Chang)
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