A new report from The Wall Street Journal highlights an evolving threat in the ongoing civil war in Yemen: cheap drones outfitted with explosives used to attack government forces.

Since 2015, Yemen’s government has been locked in a civil war against the Houthis, a militia movement that originated in Northern Yemen. That conflict has become incredibly controversial. It’s largely seen as part of a larger regional conflict between Saudi Arabia and Iran, it’s caused considerable casualties that have led to accusations of war crimes, and it has caused a famine that’s killed an estimated 50,000 people. The US has gotten involved, supplying weapons and personnel to the Saudi-led coalition, which has been combating the Houthi rebels.

The Wall Street Journal’s report cites sources that say the Houthi rebels have stepped up their game when it comes to using drones, graduating to using surveillance for airborne attacks. According to the report, Saudi officials estimate they’ve shot down more than 140 drones and that “their technology has quickly evolved from small, propeller-powered surveillance drones to a larger plane-shaped model, dubbed UAV-X by United Nations investigators, that can travel more than 900 miles at a speed of 150 miles-per-hour.” That’s worrisome because it greatly expands the reach of the rebel groups, and it could see the violence spread beyond Yemen’s borders to affect shipping lanes in the Red Sea.

The WSJ says that the “Houthis have become one of the most adept militant groups at using drones in war,” and that some intelligence officials think that the group has received assistance from Iran.

Drones range in size from the big, unmanned aircraft used by the U.S. military to the toys found in shopping malls. The Houthi experience shows that the technology can be acquired, smuggled into a rebel region, and weaponized.

Since last year, the Houthis have launched a series of attacks using drones. In July 2018, they claimed responsibility for an attack against an oil refinery in Riyadh, Saudi Arabia, and against the international airport in the United Arab Emirates. In January, they used a bomb-laden drone to attack a number of Yemeni military officials, killing military intelligence head Maj. Gen. Mohammad Saleh Tamah. It is thought to be the first time that a drone was used to successfully carry out a targeted attack. The group has since threatened similar attacks. While Yemen has launched its own attacks against suspected drone stockpiles, they haven’t been entirely successful, as the Houthis have continued to launch drone attacks in recent weeks, which highlights the difficulties in combating the threat these drones pose.

There’s more in the full WSJ report, which you can read right here.