The U.S. government indicated Thursday that it has not detected any clear signs that North Korea's nuclear test is around the corner.

"Nothing to point to, I mean, other than the rumors that we've heard in the press and elsewhere," Mark Toner, deputy spokesman for the State Department said at a press briefing.

He was responding to media reports that Pyongyang seems to be all set for a third nuclear experiment. It conducted underground nuclear tests in 2006 and 2009.

Toner stressed that North Korean leadership should make a choice between dialogue and isolation.

"North Korea has a clear choice in front of it," he said. "And if it continues its bad behavior, it's just only going to further isolate itself."

Citing unidentified sources, some media reported that the North may detonate a nuclear device as early as this week.

Toner added he "can't speak to any intelligence" that the U.S. might have on the issue.

He also refused to go into details on another sensitive topic -- South Korea's push to bolster its ballistic missile capabilities.

The South's President Lee Myung-bak openly declared a need to develop longer-range ballistic missiles to counter the North's threats.

Under a deal with the U.S., South Korea can't have ballistic missiles with a range longer than 300 kilometers.

Seoul claims that it needs longer-range missiles to cover the whole North Korean region.

But the U.S. is more concerned about a regional arms race, according to sources.

"We are strongly in support of the defense of South Korea, and that goes without saying, that we seek to work productively and constructively with South Korea on meeting their security needs," Toner said in reiterating Washington's basic stance.

"But I don't have a particular comment on that story," he added.

He hinted that the U.S. will focus more on implementing U.N.-led sanctions on the North for its recent long-range rocket test, rather than seeking bilateral ones.

"This is a heavily sanctioned country to begin with, so part of the calculus here is to not only seek new entities to sanction, but to also strengthen enforcement of existing sanctions," he said.

Manwhile, five nuclear powers -- the U.S., China, Russia, France, Britain -- delivered a unified message for North Korea to drop a plan, if any, for a nuclear test.

North Korea should "refrain from further actions which may cause grave security concerns in the region, including any nuclear tests," they in a statement on the sidelines of a non-proliferation meeting in Vienna.

(Yonhap News)