North Korea may be preparing to launch a long-range missile as soon as in a week, Japan's Kyodo news agency reported early on Thursday, citing an unnamed Japanese government official.
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 U.N. Security Council members were discussing fresh sanctions against the North after it conducted its fourth nuclear test on Jan. 6. The North is already under sanctions for its nuclear and missile programmes, Reuters reported.
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 (ICBM).
The North is also seen to be working to miniaturise 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.
On Wednesday in Beijing, U.S. Secretary of State John Kerry and Chinese Foreign Minister Wang Yi agreed on the need for a significant new U.N. Security resolution against the North but there were few signs of concrete progress.
U.S. Navy Admiral Harry Harris, commander of U.S. 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 United States.
He said he supported reviewing the possibility of converting a U.S. Aegis missile defense test site in Hawaii into a combat-ready facility to bolster U.S. 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 Area Defense in South Korea.
That decision must be made jointly by the United States and South Korea, he said.
North Korea said on Jan. 6 it exploded a hydrogen bomb, although the United States and other governments and experts voiced scepticism that it had made such a technological advance.