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INDIAN SPRINGS, NV - NOVEMBER 17: An MQ-9 Reaper remotely piloted aircraft (RPA) is parked in an aircraft shelter at Creech Air Force Base on November 17, 2015 in Indian Springs, Nevada. The Pentagon has plans to expand combat air patrols flights by remotely piloted aircraft by as much as 50 percent over the next few years to meet an increased need for surveillance, reconnaissance and lethal airstrikes in more areas around the world. (Photo by Isaac Brekken/Getty Images)
CNN  — 

An unmanned US military drone was shot down off the coast of Yemen by Houthi forces, a defense official told CNN.

The official said the MQ-9 Reaper drone was operating in international airspace and over international waters when it was shot down. US Central Command is investigating the incident, the official said.

The shootdown, which the Iran-backed Houthi militants took responsibility for earlier on Wednesday, comes several weeks after a US Navy warship, the USS Carney, intercepted multiple missiles and drones fired by the Houthis as they were heading north along the Red Sea. The ship shot down four cruise missiles and 15 drones over a period of nine hours, CNN previously reported, and their trajectory left little doubt that the projectiles were headed for Israel.

Iranian proxy groups have stepped up their attacks on US forces and assets in the Middle East in recent weeks following Hamas’ attack on Israel. US and coalition bases in Iraq and Syria have been attacked at least 40 times since October 17, leaving multiple US servicemembers with traumatic brain injuries and other injuries, all of which have been minor, officials said.

A senior defense official called Iran “the center of gravity” for the attacks on US forces in the region, saying that “Iranian fingerprints are all over this.”

The US retaliated against the groups on October 26, launching airstrikes in Syria targeting infrastructure used by the Islamic Revolutionary Guard Corps and its proxies. The strikes, carried out by an F-15 fighter jet and a pair of F-16 fighter jets using precision-guided munitions, targeted a weapons and ammunition storage facility in Abu Kamal near the border between Syria and Iraq.

CNN has reported that Iran is seeking to capitalize on the backlash to US support for Israel, and while Tehran may not be explicitly directing the groups’ attacks, it does appear to be encouraging them.

Following last month’s retaliatory strikes in Syria, US Defense Secretary Lloyd Austin issued a strong warning to Tehran.

“Iran wants to hide its hand and deny its role in these attacks against our forces. We will not let them,” Austin said in a statement that promised further strikes if the attacks by Iran’s proxies continue.