The US knows Israel will retaliate after, and even says it supports such a move, but is trying to influence the nature of the response, warning against targeting Tehran's nuclear facilities.
US President Joe Biden, who consulted with his G7 counterparts on Wednesday after the Iran set a clear red line for Israeli Prime Minister Benjamin Netanyahu.
"All seven of us agree that they have a right to respond, but they should respond in proportion," Biden said, without going into detail about what such a response might look like.
All sides are aware that any retaliatory strike against Iran's nuclear or oil installations would send the Middle East spiralling even more towards chaos, but Israel seems not to have made a decision yet.
US Secretary of State Antony Blinken conferred with his French, British, German and Italian counterparts to convey that message, the State Department said.
In Washington and elsewhere, a sinking feeling of dread is prevailing as officials wait to see if Netanyahu, under pressure at home to act, will be tempted to strike hard and deeper into Iran, taking the fight to his country's sworn enemy.
After dealing the Iran-backed Hezbollah a serious blow in Lebanon by a, and after subduing Hamas in the Gaza Strip, will Netanyahu see this as a unique opportunity to go big? It's the million-dollar question.
On Wednesday, former Israeli premier Naftali Bennett called for a decisive strike to destroy Iran's nuclear facilities.
US State Department spokesman Matthew Miller said Washington has "made clear for some time that we do not want to see a full-blown regional escalation".
"Israel has a right to respond... and we are going to continue to discuss with them what that response might look like, but we don't want to see any action that would lead to a full-blown regional war," he told reporters.
