WASHINGTON, July 19 — American and Asian officials with access to the latest intelligence on North Korea say strong evidence has emerged in recent weeks that the country has built a second, secret plant for producing weapons-grade plutonium, complicating both the diplomatic strategy for ending the program and the military options if that diplomacy fails.

The discovery of the new evidence, which one senior administration official cautioned was "very worrisome, but still not conclusive," came just as North Korea declared to the United States 11 days ago that it had completed reprocessing 8,000 spent nuclear fuel rods, enough to make a half dozen or so nuclear weapons.

American officials have said they cannot verify that claim, though they confirm that sensors set up on North Korea's borders have begun to detect elevated levels of krypton 85, a gas emitted as spent fuel is converted into plutonium.

What concerns American, South Korean and Japanese analysts, however, is not simply the presence of the hard-to-detect gas but its source. While American satellites have been focused for years on North Korea's main nuclear plant, at Yongbyon, the computer analyses that track the gases as they are blown across the Korean Peninsula appeared to rule out the Yongbyon reprocessing plant as their origin. Instead, the analysis strongly suggests that the gas originated from a second, secret plant, perhaps buried in the mountains.

American officials have long suspected that North Korea would try to build a second plant to protect itself against a pre-emptive strike by the United States. The United States even demanded an inspection of one underground site five years ago, only to find it empty, but this is the first time evidence has emerged that a second plant may be in operation. [...]