A statement says "rescue efforts are ongoing" after an incident involving two planes, the second of which landed safely.BBC reported.
Earlier, Israel said it had launched a new wave of strikes against the Iranian capital Tehran - it has also targeted Hezbollah in Lebanon today
Elsewhere, a statement attributed to Iran's new supreme leader was read out on state-controlled TV - it is his first since assuming power and included a commitment to continue blocking the Strait of Hormuz
Mojtaba Khamenei has not been seen in public since succeeding his father. There is speculation he has been injured or even killed, says BBC Persian
At least three more cargo ships have been attacked in the Gulf, apparently by Iran. Iranian state media say Iranian forces were behind at least one strike
The UN Refugee Agency says 3.2 million people have been displaced inside Iran since Israel and the US launched their attacks on 28 February
We do not know what brought down this KC-135 refuelling aircraft but one possibility is a mid-air collision.
These tankers play a major role in wars, keeping fighter jets and bombers properly fuelled for combat when such aircraft have to fly long distances to get to target.
It's a huge logistical operation, with many jets in the air at the same time, each requiring fuel.
Refuelling involves flying very close to the tanker, before a probe from the tanker is extended and lowered.
The receiving aircraft pilot flies up to the probe and, using a system of lights under the tanker, re-positions themselves so the probe can literally "plug in" to the jet.
Once contact is made, fuel is transferred in a process that can take several minutes.
All this time the aircraft is literally feet from the larger tanker - in operations that often happen at night.
It requires considerable skill for the pilot to maintain contact with the probe, or in some cases a shuttlecock-shaped drogue, in an environment where there may be many other planes close by.
And during refuelling missions, lights on aircraft may be out completely to avoid detection by the enemy.
US Central Command said this happened over Iraq but we don't know how far it was from neighbouring Iran.
The KC-135 typically has a crew of three - a pilot, a co-pilot and a boom operator.
The Air Mobility Command, according to the USAF, has nearly 400 tankers in its fleet.
