The United Nations' nuclear inspectors declared for the first time Thursday that they had extensive evidence of "past or current undisclosed...

WASHINGTON — The United Nations’ nuclear inspectors declared for the first time Thursday that they had extensive evidence of “past or current undisclosed activities” by Iran’s military to develop a nuclear warhead, an unusually strongly worded conclusion that seems certain to accelerate Iran’s confrontation with the U.S. and other Western countries.

The report, the first under the new director of the International Atomic Energy Agency (IAEA), Yukiya Amano, also concluded that Iran’s weapons-related activity continued “beyond 2004,” contradicting a U.S. intelligence assessment published about two years ago that said work on a bomb was suspended at the end of 2003.

The report confirms that Iran has enriched small quantities of uranium to 20 percent but makes no assessment of how close it might be to producing a nuclear weapon, which Iran denies it is seeking to do.

The U.N. report cited new evidence that appeared to paint a picture of a concerted drive in Iran toward a weapons capability. The agency described an escalating series of steps by Iran: the enrichment to 20 percent, its acknowledgment of a secret enrichment plant in Qum, its efforts to metalize uranium and its rejection of a deal to enrich its uranium outside the country.

The report also reiterated evidence that Iran appeared to have tested ways of detonating weapons and to have worked to design warheads small enough to fit atop a missile.

One senior Obama administration official said he thought the actions described in the report “almost suggest the Iranian military is inviting a confrontation.”

Some in the Obama administration suspect that Iran’s Islamic Revolutionary Guard Corps or its leading mullahs are betting that an escalation of the nuclear confrontation might distract attention from the street protests that have rocked the government.

The report buttressed that view by indicating that Iran had moved most of its stockpile of low-enriched uranium into an aboveground storage facility at Natanz, where it is vulnerable to military attack.

“It’s odd, and there is no technical explanation for it,” the senior administration official said at a briefing. “There must be some other explanation.”