In recent weeks the North Koreans have appeared more conciliatory, including expressing a willingness to return to negotiations with the United States, China, Russia, South Korea and Japan, though they have not clarified if they would be willing to agree to the ultimate goal of nuclear disarmament. The United States and its allies, however, have stood firm, saying they would resume the talks on the nuclear program only if the North agreed to eventually give up its arsenal.

Analysts said the re-emergence of steam at the plant — whether real or a contrivance — could well be designed by the North to try to force the United States and its allies into the talks in the hopes of winning economic aid by creating the fear that North Korea is preparing to add to its weapons stockpile.

But the move could have the opposite effect. President Obama has been deeply reluctant to take steps that would reward North Korea for halting activities it had already agreed to stop. His former secretary of defense, Robert M. Gates, once famously said that the United States was “tired of buying the same horse twice.” So far there have been no new offers to the North Koreans, and with attention focused on Syria and the Arab Spring uprisings, the administration has been reluctant to spend much political capital on a country that seems unlikely to give up its arsenal.

David Albright, the president of the Institute for Science and International Security, a Washington research group that tracks the North Korean nuclear program, said he had also studied satellite images of the complex and concluded that the venting steam suggested that the reactor had indeed begun working again.

“It implies that the reactor is restarted, but that needs to be confirmed,” he said in an interview. “You want to get confirmation because you never know” with the North Koreans. “They can surprise you,” he added, “but I can’t think of any alternative explanations.”