Suspicious activities have been continuing in a rugged area of North Korea where the communist state carried out its first nuclear test this month, news reports said on Saturday.
A new structure for an unknown purpose also sprang up in Punggyeri in the north-eastern county of Kilju, the Joongang daily and Yonhap news agency said, quoting military sources.
"There have been continuous activities in Punggyeri since the nuclear test on October 9," a military source was quoted as saying by Yonhap.
"However, it remains unclear whether these activities are related with a second nuclear test or North Koreans are just faking it," the source said.
A defence ministry spokesman declined to comment on the reports.
The Science and Technology Ministry said Punggyeri, some 350 kilometres (219 miles) north-east of Pyongyang, was believed to be the place where the nuclear test was carried out.
Earlier reports said the test appeared to have been conducted at Hwadaeri, some 30 kilometres south-west of Punggyeri.
Punggyeri is where vehicle movements and the unloading of large reels of cable were spotted by satellite images last month, prompting speculation that a nuclear test was being prepared.
Chung Hyung-Keun, an opposition Grand National Party lawmaker who serves on parliament's intelligence committee, has said that the first nuclear test was carried out in one of two horizontal tunnels dug into hills in Punggyeri.
"A shelter was built outside the other tunnel and some 30 to 40 people have been working there," Chung said on a radio talk show on Thursday last week.
The October 9 test caused a global uproar with the UN Security Council issuing a resolution imposing economic sanctions aimed at reining in Pyongyang's weapons programme.