The U.N.'s atomic watchdog and other experts say radiological contamination is likely inside a key uranium enrichment site at Natanz that was badly damaged by Israel and then hit by U.S. strikes early Sunday.

The move, the first time the U.S. has attacked Iran, is a major escalation of the conflict between Israel and Iran.

Ballistic experts say it's too soon to determine how much damage Iran's nuclear facilities sustained in the weekend attack. On Saturday night, the United States targeted nuclear sites in Iran believed to be essential to the country's ability to produce nuclear weapons: a uranium enrichment site located in northwestern Iran -- deep inside a mountain in Fordo -- along with another uranium enrichment site in Natanz and the Isfahan Nuclear Technology Center,