Seoul, Dec 21 (DPA) South Korea said North Korea must return to the international nuclear Non-Proliferation Treaty (NPT) before any visit to its nuclear facilities by UN inspectors, a news report said Tuesday.

The statement came a day after Pyongyang told a visiting US official that it would allow international inspectors back into its Yongbyon nuclear complex.

But a South Korean official, quoted by the Yonhap News Agency, dismissed the invitation as "an old trick".

"North Korea may simply be trying to receive justification for its uranium enrichment programme," the official was quoted as saying.

"If North Korea is serious about international inspection, it must first rejoin the NPT," the official said. "And in order to return to the NPT, all nuclear programmes must be frozen and withdrawn."

Pyongyang left the NPT, which prohibits its non-nuclear-armed members from acquiring or developing nuclear weapons, in 2003. It successfully tested nuclear bombs in 2006 and 2009.

Concerns have risen in recent weeks amid increasing tensions between North and South Korea, and as intelligence reports have indicated the presence of a previously unknown uranium-enrichment facility at Yongbyon in the west of the country.

Last month, North Korea responded to an artillery drill near an island in disputed waters by shelling the island, killing two South Korean soldiers and two civilians.

On Monday, South Korea live-fire exercises off the same island went ahead despite threats of retaliation by the North, but were concluded without response.

Bill Richardson, a former US ambassador to the UN and currently governor of New Mexico, said Tuesday that North Korea had shown "a certain pragmatism" by not retaliating to Monday's drills.

Speaking in Beijing after visiting Pyongyang Monday, Richardson said the mood seemed ripe for talks, according to the Yonhap report. North Korean officials had told him that they would consider welcoming inspectors of the UN's International Atomic Energy Agency.

The official quoted by Yonhap was dismissive of the possibility of resolving the tensions through diplomacy. "North Korea has taken lives and has threatened (war)," he said. "We must be able to answer force with force, and then we can have talks as necessary."

Pressure has also been mounting on Pyongyang to return to the six-nation talks to end its nuclear programme. North Korean leaders walked out of the talks, which include Japan, Russia, China, South Korea and the US, in April 2009 over renewed UN Security Council sanctions.