US Secretary of State Condoleezza Rice said last night speculation about an Israeli attack on Iran's nuclear facilities underscored the risk of failing to persuade Tehran to halt uranium enrichment.
Dr Rice said she believed there was still "plenty of room for diplomacy" in curbing an Iranian nuclear programme that Western powers fear could lead to making bombs.
Asked for her view of an Israeli military operation in Iran should diplomacy fail, she replied: "Well, I think that even talk of such just shows how very serious it would be to have Iran continue its programs unabated."
Dr Rice was in Israel and the Palestinian territories as part of a regional visit to explore the chances of reviving Israeli-Palestinian peacemaking and to seek Arab help to stabilise Iraq.
It was reported a week ago that Israel had drawn up secret plans to destroy Iran's uranium enrichment facilities with tactical nuclear weapons.
Israel has refused to rule out pre-emptive military action against Iran along the lines of its 1981 air strike against an atomic reactor in Iraq, although many analysts believe Iran's nuclear facilities are too much for Israel to take on alone.
Washington has said military force remains an option while insisting that its priority is to reach a diplomatic solution.
The UN Security Council voted unanimously last month to impose sanctions on Iran to try to stop its uranium enrichment. Tehran insists its plans are peaceful and says it will continue enrichment.
Iranian President Mahmoud Ahmadinejad has called for Israel to be "wiped off the map". Israel, widely believed to have the Middle East's only nuclear arsenal, has said it will not allow Iran to acquire nuclear weapons.