A new US assessment has found that US strikes in June mostly destroyed one of three targeted Iranian nuclear sites, but the other two were not as badly damaged, NBC News reported on Thursday, citing current and former US officials.

The report said that US officials believe the attack on Iran's Fordow nuclear facility was successful in setting back enrichment capabilities there by as much as two years, citing two current officials.

The two other facilities that the US struck were not as badly damaged and may have been degraded only to a point where nuclear enrichment could resume in the next several months if Iran wants it to, the report added.

Reuters could not immediately verify the NBC report. The White House did not immediately respond to a Reuters request for comment.

White House spokesperson Anna Kelly told NBC News in a statement: "As the President has said and experts have verified, Operation Midnight Hammer totally obliterated Iran's nuclear capabilities.”

The chief Pentagon spokesperson Sean Parnell told NBC that US President Donald Trump "was clear and the American people understand: Iran's nuclear facilities in Fordow, Isfahan, and Natanz were completely and totally obliterated. There is no doubt about that."

The US launched strikes on Iran's nuclear facilities last month, saying that they were part of a program geared towards developing nuclear weapons. Tehran maintains that its nuclear development is purely for civilian purposes.

A preliminary assessment in June 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.