The U.S. and its allies worked to head off an Iranian attack on Israel and avert a wider regional war as concerns grew that a strike may come at any moment in retaliation for the killing of a top Hamas leader in Tehran.

The Biden administration moved additional forces to the region, and Secretary of State Antony Blinken conferred with top officials from Qatar and Egypt — the two countries helping lead negotiations for a cease-fire between Israel and Hamas militants — on Monday, according to State Department spokesman Matthew Miller.

"It is a critical moment,” Blinken told reporters in brief comments Monday in Washington. "We are engaged in intense diplomacy — pretty much round the clock — with a very simple message: All parties must refrain from escalation, all parties must take steps to ease tensions.”