North Korea continues to keep the experts guessing. Last week, it promoted the third son of its current leader, Kim Jong Il, prompting speculation that he is on track to succeed his father. And now, apparently, it has commissioned construction activity at the site where it used to produce plutonium for its nuclear arsenal.

An image taken last week by DigitalGlobe, a U.S.-based commercial satellite firm, shows new construction or excavation activity in an area surrounding a destroyed cooling tower at the Yongbyon site. Experts said the construction appeared to be the first sign of genuine activity at Yongbyon since 2008, when the cooling tower was demolished as part of an agreement made during now-stalled negotiations over the North's nuclear program.

The photograph shows heavy machinery tracks, trucks, and heavy construction or excavation equipment, along with two small new buildings, according to a report by the Institute for Science and International Security. What exactly all that activity means, said David Albright, who wrote the report, is unclear.

Albright said the activity could mean that North Korea is moving toward reopening Yongbyon as part of a plan to increase its stock of plutonium - now estimated at just less than 80 pounds. Then again, it could also be a move, said Joel S. Wit, a North Korea watcher and former State Department official, "for show, to pull our chains." North Korea is well aware that its nuclear facilities are under almost constant surveillance by both intelligence and commercial satellites.

The photograph was released as the United States and its partners in the region try to figure out how to deal with North Korea after the sinking of a South Korean warship in March, which Seoul has blamed on Pyongyang.

In recent weeks, the South and the North have reached out to each other. South Korea has sent some food aid. North Korea has allowed families divided after the Korean War to meet. The United States has also conducted a review of its policy of "strategic patience."

North Korea, which has been referring to itself as a "nuclear weapons state," has been vowing for months to resume some type of nuclear activity. Now, the new satellite image has experts worried that North Korea is carrying out its pledge.

"It is by no means clear what is happening," said Jonathan Pollack, an expert on North Korean security at the U.S. Naval War College, "but any new construction at Yongbyon cannot be a good thing."

Pollack said North Korea has succeeded in getting the United States' attention before. In October 2006, it tested a nuclear device, prompting the administration of George W. Bush to resume talks with Pyongyang. It conducted a second nuclear test in May 2009.

Pollack said any resumption of activity at Yongbyon could be a payoff of sorts for North Korea's military. The North appears to be in the throes of a leadership transition, with the elder Kim engineering promotions for his third son, Kim Jong Eun.

"It could be a compensation package for the military," Pollack said.

Both Pollack and Wit stressed that U.S. officials would underestimate North Korea's capabilities at their own peril.

"It's a serious mistake," Wit said, "to believe that they are not capable of doing anything to step up their nuclear arsenal."