North Korea may be preparing to launch a long-range missile as soon as within a week, Japan's Kyodo news agency reports, 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 UN Security Council members discuss fresh sanctions against the North after it conducted its fourth nuclear test on January 6. The North is already under sanctions for its nuclear and missile programs.
North Korea last conducted a long-range rocket launch in late 2012, successfully putting into orbit an object it claimed was a communications satellite, in what experts saw as part of an effort to build an intercontinental ballistic missile (ICBM).
South Korean defence ministry spokesman Kim Min-seok declined to comment on whether there were pre-launch activities at the site, citing a policy of not discussing intelligence matters. However, he said the North had issued no international warnings on navigation, as it has ahead of previous long-range rocket launches.
South Korea's Yonhap news agency cited a government source as saying there had been steady activity at the missile base, with screens set up at key areas, probably to deter spy satellite surveillance.
Much of the site's operation is automated and rails are set up to move rocket components quickly for final assembly and launch, Yonhap quoted the source as saying.
The site was upgraded last year to accommodate the launch of a longer-range rocket, experts have said.
Isolated North Korea says it has a sovereign right to run a space program and its long-range rockets are built to deliver satellites into space.
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.
In Beijing, US Secretary of State John Kerry and Chinese Foreign Minister Wang Yi on Wednesday agreed on the need for a significant new UN security resolution against the North, but there were few signs of progress.
US Navy Admiral Harry Harris, commander of US Pacific Command, said before publication of the Kyodo report that he supported reviewing the possibility of converting a US Aegis missile defence test site in Hawaii into a combat-ready facility to bolster US defences 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 defence system known as the Terminal High Altitude Area Defence in South Korea.
That decision must be made jointly by the United States and South Korea, he said.
North Korea said on January 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.