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In Yemen, the high-pitched whine of drones has been a part of life for over 15 years, ever since the first U.S. drone strike here targeting al-Qaida in 2002. But now, Iran-backed Houthi rebels increasingly deploy drones in Yemen's brutal civil war.

Neighboring Saudi Arabia, which has been battling the rebels since 2015, said drones attacked an oil pipeline, targeting two pumping stations west of its capital, Riyadh, on Tuesday.

The Houthis claimed a coordinated drone attack, underscoring how the Arab world's poorest country has become one of the world's top battlefields for drones. Both the rebels and the Saudi-led coalition fighting them, as well as the U.S., continue to use them for surveillance and attacks.

While the U.S. uses American-made drones and the coalition has turned to Chinese suppliers, the manufacturer of the Houthis' drones in both the air and the sea has been a contentious question.

Here are some key details about the rebels' drones:

THE IRAN LINK

A 2018 report by a United Nations panel of experts on Yemen looked particularly at the Houthis' Qatef-1 drone.

The report said that although the rebel media announced the Houthis had manufactured the drone, "in reality they are assembled from components supplied by an outside source and shipped into Yemen."

The Qatef, or "Striker," it added, "is virtually identical in design, dimensions and capability to that of the Ababil-T, manufactured by the Iran Aircraft Manufacturing Industries."

The Ababil-T can deliver up to a 45-kilogram (100-pound) warhead up to 150 kilometers (95 miles) away.

A research group called Conflict Armament Research, with the permission of the United Arab Emirates' elite Presidential Guard, also examined seized drones used by the Houthis and their allies to crash into Patriot missile batteries in Saudi Arabia.

The research group similarly said those drones share "near-identical design and construction characteristics" of Iranian drones.

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THE DRONE BOAT

Saudi-led coalition forces last year also showed journalists a Houthi "drone boat," filled with explosives that had failed to detonate.

The officials also shared black-and-white images they said came from the "drone boat." They said the pictures and associated data from the boat's computer showed Iranians building components for its guidance system in eastern Tehran, with a hat in the background of one picture bearing the symbol of Iran's hard-line paramilitary Revolutionary Guard forces.

They said those involved in building the components probably believed it would be destroyed in the blast, so they didn't wipe the computer's hard drive

For its part, Iran repeatedly has denied supplying the Houthis with drone or ballistic missile technology. However, Iran would have an interest in seeing Saudi Arabia, its archrival in the region, tied down in a bloody, protracted conflict with no clear end in sight.

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Follow Jon Gambrell on Twitter at www.twitter.com/jongambrellap .