North Korea may have built more underground nuclear test sites in the northeastern district where it staged its first two tests, a news report said on Monday.
South Korean intelligence sources quoted by Yonhap news agency said the North could have built two or three such sites in and around Punggyeri in Kilju district near the coast.
US intelligence sources quoted by American TV networks said last week the North intends to respond to new UN sanctions with a third nuclear test.
"There are no signs yet of preparations for a third test," a source told Yonhap.
JoongAng Ilbo newspaper, quoting intelligence sources, said South Korean and US officials have intensified satellite monitoring of 11 underground facilities for a possible test.
It said some sites are in the north of the country and include Kumchang-ri in the northwest, which came under suspicion back in 1998 as a possible hidden atomic facility.
The United States gave the North 600,000 tons of food aid in return for permission to inspect the site but US visits in 1999 revealed only empty tunnels.
An intelligence source told AFP that the North's activities are being closely monitored but it was not true that 11 sites were being watched.
"It's not easy to pick a multiple number of possible nuclear test sites and closely monitor all of them," one official told Yonhap.
"In 2006 we made a list of suspected North Korean nuclear facilities for possible verification. But we cannot just conclude that these facilities are all possible nuclear test sites."
The National Intelligence Service declined to comment on the media reports.