North Korea (search) has denied reports that it might be preparing to test a nuclear weapon, calling them a U.S. "fabrication," South Korea's Yonhap news agency said Friday.

U.S. officials said earlier this month that spy satellites had detected tunnels and a reviewing stand under construction in northeastern North Korea and suggested they were preparations for a nuclear test. North Korea responded by criticizing the United States but did not deny such preparations were under way.

"The U.S. leadership has recently ... come out with a fabrication that there are some kind of missile tests and signs of an underground nuclear test," Yonhap quoted the Korean Central Television Station (search), the North's only nationwide network, as saying Thursday.

KCTS said the United States continued to use "very bellicose, abusive language" toward North Korea.

The North has in recent days stepped up its anti-U.S. rhetoric, repeating claims that Washington is preparing an attack to unseat North Korean leader Kim Jong Il (search) and his government, and refusing to rule out a pre-emptive attack by Pyongyang instead.

But there also has been a recent flurry of activity aimed at persuading Pyongyang (search) to return to six-party talks, stalled since the third round ended last June, over its nuclear weapons program. It's not unusual for the North to raise tensions before entering into negotiations in hopes of extracting aid and other concessions from the West.