The United States has received indications North Korea (search) might be trying to test a nuclear weapon (search), a senior Bush administration said Sunday.

The official said there is no evidence that a large mushroom cloud (search) that reportedly billowed up from North Korea was linked to the communist nation's suspected nuclear weapons program.

"We're watching the indicators to see whether this is normal activity or whether something else is under way," the official said on condition of anonymity.

The White House periodically receives reports that North Korea is seeking to test a nuclear weapon, the official said.

President Bush's (search) senior foreign policy advisers said publicly Sunday that they did not think the reported explosion Thursday near North Korea's border with China was related to North Korea's nuclear aspirations.

"We have no indication that it was a nuclear event of any kind," Secretary of State Colin Powell told ABC's "This Week." "Exactly what it was, we're not sure."

He said the administration was closely watching activities taking place at some sites in North Korea, but that "it is not conclusive that they are moving toward a test."

National security adviser Condoleezza Rice said on a cable news network that it would not be "smart" for the North Koreans to test because it would further isolate them.

Rice also said the explosion was not likely a test. "We don't think, at this point, it was a nuclear event, but we're looking at it and will get further analysis," she said. "There are all kinds of reports and all kinds of assessments that are going on. Maybe it was a fire — some kind of forest fire."

Asked whether a U.S. military option is on the table concerning North Korea, Rice said, "The president never takes any option off the table, but we believe the way to resolve this is diplomatically."

The United States, Russia, Japan, China and the two Koreas have held talks on North Korea's suspected nuclear weapons development, and they agreed to hold another round of negotiations in Beijing (search) this month. No date has been set.

The United States has pushed for North Korea to fully disclose all of its nuclear activities and allow outside monitoring before it receives any assistance. North Korea wants energy aid, lifting of economic sanctions and removal from its inclusion on Washington's list of state sponsors of terrorism.

"North Korea is looking for assurances that we're not going to invade it, we have no hostile intent; they're looking for benefits for giving up their nuclear capability and their nuclear infrastructure. And what we're debating is what will it take to give them the assurances they need and what benefits would they expect over the long haul," Powell said.

But, he said, the United States will not "reward them for doing something they should've have been doing in the first place. So we're into a very intense period of negotiations."