South Korea is keeping a close watch on the movement of trucks and soldiers at the Punggye-ri site in the North Korea's remote northeast, Yonhap news agency reported, citing several unidentified military officials. One official, however, said a second test was "not believed to be imminent."
"We are closely monitoring to see if these are preparations for a second nuclear test," another official was quoted as saying.
South Korea has also detected a new building being erected at the site, the JoongAng Ilbo newspaper reported, citing unidentified government officials.
Separate U.S. and South Korean studies have detected abnormal radiation in air samples, confirming the North has conducted a nuclear test. The South Korean government has pointed to Punggye-ri as a place where the North most likely conducted the underground blast.
"Intelligence agencies from South Korea and the United States are trying to confirm whether this new building is connected to another nuclear test," an official was quoted as saying.
It was not immediately clear how military officials first spotted the activities at the site. However, the United States and South Korea generally share intelligence information from satellite images.
South Korea's Defense Ministry said it could not confirm the reports.
The U.S. State Department refused to comment. Pentagon spokesman Air Force Maj. David Smith said, "We don't discuss intelligence issues as a matter of policy."
There have been several reports of suspicious activity at Punggye-ri since North Korea's Oct. 9 underground nuclear test. But South Korean officials have said they have received no intelligence reports suggesting another test is imminent.
South Korean Foreign Minister Ban Ki-moon — the incoming U.N. secretary-general — met with Chinese leaders Friday to discuss sanctions against the North over the test. South Korea's Foreign Ministry said it had no information about the outcome of the talks.
Seoul and Beijing have been reluctant to enforce a U.N. Security Council resolution that calls for sanctions on the North, fearing they might aggravate their volatile neighbor and destabilize the region.
China and South Korea are the North's main aid providers and trade partners, and their participation is considered crucial for the success of the U.N. resolution, which bans the sale of major arms to the North and calls for the inspection of cargo entering and leaving the country.
In a report Friday, the World Food Program warned that the U.N. resolution may deter countries from making food donations to North Korea, where millions are believed to have died of hunger in the past decade.
South Korea suspended its regular humanitarian aid of rice and fertilizer to its impoverished neighbor after the North test-fired a barrage of missiles in July. Supplies from China have also shrunk to one-third of last year's levels, the WFP has said.
The United States, meanwhile, reiterated its position that it will not negotiate with the North until the reclusive state returns to six-nation talks on its nuclear ambitions.
North Korea has refused to return to negotiations unless Washington lifts financial restrictions imposed on Pyongyang. It has also been pushing for bilateral talks with Washington.
State Department spokesman Sean McCormack said Friday that Washington would be willing to hold one-on-one talks with North Korea only if it returns to the six-party negotiations, which also involve South Korea, China, Russia and Japan.