As Iran retaliates, largest US military base in Middle East hit by ballistic missile, Qatar says

Secretary of State Marco Rubio said the "intensity" of the U.S.-Israeli attacks on Iran will ramp up in the coming hours and days.

"You're going to really begin to perceive a change in the scope and the intensity of these attacks as, frankly, the two most powerful air forces in the world take apart this terroristic regime and defang it and take away its ability to threaten its neighbors or hide behind the zone of immunity that allows them to develop nuclear ambitions," Rubio told reporters earlier Tuesday.

The top U.S. commander in the Middle East said the U.S. has "severely degraded" Iran's air defenses, destroying hundreds of Iran's ballistic missiles, launchers and drones.

"In simple terms: We’re focused on shooting all the things that can shoot at us," Admiral Brad Cooper said in a video update on the operation released Tuesday.

Cooper said the U.S. has also destroyed 17 Iranian ships, including an Iranian submarine.

More than 50,000 troops are participating in the operation, which has so far struck nearly 2,000 targets in Iran, he said.

"Many of you may remember the Shock and Awe strikes of 2003. The first 24 hours of this operation were nearly doubled the scale," he said.

Cooper referenced the first U.S. use of one-way drones, saying, "I'd like to point out these drones were originally an Iranian design. We took them back to America, made them better and fired them right back at Iran."

Cooper said the U.S.' combat power is "building" while Iran's is "declining."

"My overall operational assessment is that we are ahead of our game plan," he said.

-ABC News' Steven Benyon

More than 9,000 Americans have returned from the Middle East since the launch of the joint U.S.-Israeli strikes on Iran, according to President Donald Trump.

This number reflects Americans who left on their own volition in recent days without the State Department assistance that is now being offered.

Trump said in a social media post that if there are other U.S. citizens who wish to return home from the region, they should register with the State Department.

"The Department will identify where you are, and provide travel options to you," Trump said. "We are already chartering flights, free of charge, and booking commercial options, which we expect will become increasingly available as time goes on."

-ABC News' Mariam Khan and Nicholas Kerr

The Pentagon has identified four of the six American troops killed in the opening hours of the war with Iran.

Capt. Cody A. Khork, 35, of Winter Haven, Florida; Sgt. 1st Class Noah L. Tietjens, 42, of Bellevue, Nebraska; Sgt. 1st Class Nicole M. Amor, 39, of White Bear Lake, Minnesota; and Spc. Declan J. Coady, 20, of West Des Moines, Iowa, died Saturday in Kuwait from an Iranian drone attack.

All soldiers were assigned to the 103rd Sustainment Command, an Army Reserve unit based in Des Moines, Iowa.

-ABC News' Steven Beynon