By Barbara Starr and Elise Labott

The United States believes North Korea is ready "at any time" to conduct a nuclear test, American officials tell CNN.

"We think they are preparing for a test," one U.S. official told CNN Wednesday. "We are watching it all as closely as we can."

U.S. officials say they are bracing for a third test by Pyongyang "soon," although they caution it's near impossible to predict the timing.

READ: How close is North Korea to a nuclear missile?

"One thing for sure," a senior official told CNN. "They will definitely test a nuclear weapon. But the tricky thing is, nobody can tell you for your day planner that is when it will be."

One official explained the United States is not certain about the timing of a test because it would happen underground and the final preparations can't be observed by satellite.



But a level of activity has been seen at the site in recent days, including movement of people and equipment that typically would be expected ahead of a test, one of the officials said.

Last week, North Korea said that it planned to conduct a new nuclear test and carry out more rocket launches after the U.N. Security Council voted to tighten sanctions on the secretive regime. The U.N. action was in response to North Korea's successful launch of a three-stage rocket last month that put the satellite Shining Star-3 into orbit on the first anniversary of Kim Jong Il's death.

The launch also signaled that the North's long-range missile program now puts the United States within reach.

"If you go by what little activity we see and by their rhetoric, which is usually consistent with their actions, it's a good bet they will do (a test) soon," the senior official said. "The trash talk has reached such a crescendo that by their own ego, we don't see how they back down. But let me be clear, the physical signs don't show imminence, because the major activity is underground."

Declaring the sanctions to be tantamount to "a declaration of war," North Korea's threats of more missile and nuclear tests are part of what it said is a new phase of confrontation with the United States.

Pyongyang didn't say when it intends to carry out the nuclear test, which follows previous underground detonations in 2006 and 2009.

Satellite imagery shows activity at the Punggye-ri site, where those previous nuclear tests were conducted. The images were analyzed by 38 North, a website maintained by researchers at the U.S.-Korea Institute at John Hopkins University. According to their analysis, the roads surrounding the site have been kept clear of snow for the past month and suggest that the North Koreans may have been sealing the tunnel into a mountainside where a nuclear device would be detonated.

Officials say they, too, have also seen the imagery, but say it does not indicate whether the activity is a "real" intention to test, or how soon it could be done.

The administration is not sure if Pyongyang's announcement threatening further tests is an official announcement of an impending launch or a preliminary statement to be followed by announcement of a window of time during which the test will be conducted.

Even announcements of such windows are not always reliable. Last month North Korea extended its original window for the rocket launch, only to launch it before the new time frame.

"The North Koreans are experts at keeping the world guessing," one senior official said. "It's really impossible to tell beforehand" when they will act.

The North Korean's recent launch of a satellite into space caught U.S. intelligence somewhat off guard in that they were under the belief North Korea had delayed the launch a bit for technical reasons.

It is believed that North Korea put out the story of technical issues in order to throw off monitoring of its activities.

But knowing the exact timing of the nuclear test, while helpful, is not critical from a strategic or tactical point of view, given the United States is not trying to physically stop the test, some in the U.S. government argue.