Washington launched the latest round of its air campaign against Yemen's Huthis in mid-March, and MQ-9s can be used for both reconnaissance -- a key aspect of US efforts to identify and target weaponry the rebels are using to attack shipping in the region -- as well as strikes.

"There have been seven MQ-9s that have gone down since March 15," the US official said on condition of anonymity, without specifying what caused the loss of the drones, which cost around $30 million apiece.

The US Navy meanwhile announced the loss of another piece of expensive military equipment: an F/A-18E warplane that fell off the USS Harry S. Truman aircraft carrier in an accident that injured one sailor.

A tractor that was towing the F/A-18E -- a type of aircraft that cost more than $67 million in 2021 -- also slipped off the ship into the sea.

"The F/A-18E was actively under tow in the hangar bay when the move crew lost control of the aircraft. The aircraft and tow tractor were lost overboard," the Navy said in a statement.

The carrier and its other planes remain in action and the incident is under investigation, the Navy added. No details of recovery work were released.

- Weeks of heavy strikes -

It is the second F/A-18 operating off the Truman to be lost in less than six months, after another was mistakenly shot down by the USS Gettysburg guided missile cruiser late last year in incident that both pilots survived.

The Truman is one of two US aircraft carriers operating in the Middle East, where US forces have been striking the Huthis on a near-daily basis since March 15.

The military's Central Command said Sunday that US forces have struck more than 800 targets and killed hundreds of Huthi fighters, including members of the group's leadership, as part of the operation.

The Iran-backed Huthis began targeting shipping in late 2023, claiming solidarity with Palestinians in the Gaza Strip, which has been devastated by a military campaign launched by Israel after a shock Hamas attack in October of that year.

Huthi attacks have prevented ships from passing through the Suez Canal -- a vital route that normally carries about 12 percent of the world's shipping traffic -- forcing many companies into a costly detour around the tip of southern Africa.

The United States first began conducting strikes against the Huthis under the Biden administration, and President Donald Trump has vowed that military action against the rebels will continue until they are no longer a threat to shipping.