By Phil Stewart and Idrees Ali

WASHINGTON (Reuters) -The Pentagon said on Wednesday that U.S. strikes 10 days ago had degraded Iran's nuclear program by up to two years, suggesting the U.S. military operation likely achieved its goals despite a far more cautious initial assessment that leaked to the public.

Sean Parnell, a Pentagon spokesman, offered the figure at a briefing to reporters, adding that the official estimate was "probably closer to two years." Parnell did not provide evidence to back up his assessment.

"We have degraded their program by one to two years, at least intel assessments inside the Department (of Defense) assess that," Parnell told a news briefing.

U.S. military bombers carried out strikes against three Iranian nuclear facilities on June 22 using more than a dozen 30,000-pound (13,600-kg) bunker-buster bombs and more than two dozen Tomahawk land attack cruise missiles.

The evolving U.S. intelligence about the impact of the strikes is being closely watched, after President Donald Trump said almost immediately after they took place that Iran's program had been obliterated, language echoed by Parnell at Wednesday's briefing.

Such conclusions often take the U.S. intelligence community weeks or more to determine.

"All of the intelligence that we've seen (has) led us to believe that Iran's -- those facilities especially, have been completely obliterated," Parnell said.

Over the weekend, the head of the U.N. nuclear watchdog, Rafael Grossi, said that Iran could be producing enriched uranium in a few months, raising doubts about how effective U.S. strikes to destroy Tehran's nuclear program have been.

Several experts have also cautioned that Iran likely moved a stockpile of near weapons-grade highly enriched uranium out of the deeply buried Fordow site before the strikes and could be hiding it.

But U.S. Defense Secretary Pete Hegseth said last week he was unaware of intelligence suggesting Iran had moved its highly enriched uranium to shield it from U.S. strikes.

A preliminary assessment last week from the Defense Intelligence Agency suggested that the strikes may have only set back Iran's nuclear program by months. But Trump administration officials said that assessment was low confidence and had been overtaken by intelligence showing Iran's nuclear program was severely damaged.

According to Iranian Foreign Minister Abbas Araqchi, the strikes on the Fordow nuclear site caused severe damage.

"No one exactly knows what has transpired in Fordow. That being said, what we know so far is that the facilities have been seriously and heavily damaged," Araqchi said in the interview broadcast by CBS News on Tuesday.

(Reporting by Phil Stewart and Idrees Ali; Editing by Lisa Shumaker)