The US Navy said it has not lost any drones over the Gulf recently, Reuters has reported.
Iran's media earlier said a US drone was captured by the naval unit of the Islamic Revolutionary Guards Corps in the last few days after flying into Iranian airspace.
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Michael Fallon has received reassurances from his US counterpart that security ties with Britain will "endure" following a vote for Brexit.
On Friday, police said it appeared the girl had shot herself in the face with the weapon.
The showers will be slow-moving, heavy and thundery at times, particularly in central and eastern areas.