Israeli leaders on Thursday delivered one of the bluntest warnings to date of possible airstrikes against Iranian nuclear sites, adding to the anxiety in Western capitals that a surprise attack by Israel could spark a broader military conflict in the Middle East.

Defense Minister Ehud Barak, speaking at a security forum attended by some of Israel’s top intelligence and military leaders, declared that time was running out for stopping Iran’s nuclear advance, as the country’s uranium facilities disappear into newly constructed mountain bunkers.