SEOUL - North Korea faces consequences if it goes ahead with a second nuclear test, but little can be done to stop it, the U.S. special envoy to the communist state said on Friday.

Stephen Bosworth said after meeting officials in Seoul that the United States was leaving the door open for dialogue, despite the fact that Pyongyang said it was useless to talk to the Obama administration because its hostile policy left it no choice but to bolster its nuclear deterrent.

If the North Koreans decide to carry out a second nuclear test, we will deal with the consequences of that. And there will be consequences, Bosworth told reporters after meeting South Korea's foreign minister.

But we can't control at this stage what North Korea does.

A South Korean official familiar with the North said there was increased activity at North Korea's known nuclear test site, suggesting Pyongyang was gearing up for a new test.

Experts said it could take a few weeks for the North, which conducted its first nuclear test in October 2006, to prepare for another one.

Politically, North Korea wants to play out its test preparations, many of which can be seen by U.S. spy satellites, as long as possible to increase its leverage in nuclear negotiations, which means it may not come for months, if at all.

Bosworth began a swing through Asia that started in Beijing on Thursday to discuss how to rein in the reclusive North after it raised regional security concerns by threatening last week to test a nuclear device.

North Korea, angered by Washington's push to punish it at the United Nations for a launching a long-range rocket, unleashed its harshest criticism of U.S. President Barack Obama's government, saying it is trying to topple Pyongyang's leaders.

The study of the policy pursued by the Obama administration for the past 100 days since its emergence made it clear that the U.S. hostile policy toward the DPRK (North Korea) remains unchanged, the official KCNA news agency quoted an unnamed Foreign Ministry spokesman as saying.

There is nothing to be gained by sitting down together with a party that continues to view us with hostility.

He added: The DPRK will bolster its nuclear deterrent as it has already clarified.

Last month, North Korea said it was quitting the six-way talks and would restart a plant that separates plutonium from spent nuclear fuel rods in response to being punished by the United Nations for the April 5 launch of a long-range rocket.

The six-way talks began in 2003 and involve the two Koreas, the United States, Japan, Russia and China.

North Korea insists it sent a satellite into orbit and had the right to do so as a part of a peaceful space program.

U.S. and South Korean officials said nothing was put into orbit and the launch was a disguised test of a long-range missile that violated U.N. resolutions.

Destitute North Korea, which conducted its only nuclear test in October 2006, later said it had resumed producing arms-grade plutonium at its Soviet-era Yongbyon nuclear plant.

It then threatened a fresh nuclear test and ballistic missile test launch unless the U.N. Security Council apologized for chastising the state and tightening existing sanctions that limit its overseas arm trade, a key source of hard currency.

Bosworth will go to Tokyo next week. State Department envoy Sung Kim will travel to Moscow, Bosworth said.