The explosive-laden quadcopter skimmed above the rooftops of southern Lebanon, navigating with precision between bombed-out buildings and along dirt roads. The drone gave its operator a clear first-person picture of its target: an Israeli tank with soldiers standing nearby.

At the top of the picture, in white letters, were two words.

“BOMB READY”

The quadcopter is a fiber-optic drone, experts say, a weapon Hezbollah has increasingly used with deadly accuracy. The drones are difficult to stop and even harder to detect, giving their operators a high-resolution view of the target without emitting any signal that could be jammed.

The drones are “immune to communication jamming, and in the absence of an electronic signature, it is also impossible to discover the location from which they were launched,” wrote Yehoshua Kalisky, a senior researcher at Israel’s Institute for National Security Studies.

In a sleekly produced Hezbollah video from Sunday, the quadcopter drone, weighing no more than a few kilograms, hits its target as the Israeli soldiers appear to be completely unaware of its approach. According to the Israel Defense Forces (IDF), the attack killed 19-year-old Sgt. Idan Fooks and injured several others. Hezbollah then launched more drones at a rescue helicopter that arrived at the scene to evacuate the wounded troops.

Fiber-optic drones are effective in their simplicity: Instead of a wireless signal that controls the drone remotely, the fiber-optic cable hardwires the drone directly to its operator.

Because fiber optic cables are so thin and light – virtually invisible to the naked eye – the cable can stretch for up to 9.3 miles (15 kilometers) or more, an Israeli military source told CNN, allowing the operator to remain a safe distance away while the drone feeds him a crystal-clear, first-person image of the target.

The IDF has relied on its technological advantage to counter drone warfare, jamming the signals and frequencies used by drone operators to control the devices in order to stop them before they reach Israeli soldiers. But without a signal, the IDF is unable to electronically interfere with the control of fiber-optic drones and also faces a greater challenge in detecting the incoming projectile in the first place.

“Beyond physical barriers like nets, there is little that can be done,” the Israeli military source said. “It’s a low-tech system adapted for asymmetric warfare.”

Fiber-optic drones first appeared in large numbers on the battlefield in Ukraine, where Russian forces used them with great effect, extending their range even further. Russia was also able to connect the drone’s fiber-optic cable to a base unit, which was then linked to a drone operator. That added connection distanced the operator from the drone itself, protecting the human and making him even harder to target. Russia’s ability to produce the drones, or unmanned aerial vehicles (UAVs), en masse meant Moscow could cut Ukrainian supply lines with drone attacks well behind the front lines.

Hezbollah’s targets are different. Israel is operating in southern Lebanon so close to its own bases that there are no substantial supply lines to target. Instead, Hezbollah’s drone operators have hunted Israeli troops in southern Lebanon and northern Israel, well within the range of the weapons.

“This is a capable system that, in the right hands with an experienced operator against a force that’s not expecting such a drone to attack, it can be quite effective,” said Samuel Bendett, an adjunct senior fellow with the Center for New American Security. “Even against a force that knows about this and is taking precautions, it can still be deadly.”

Israel believes Hezbollah imports the civilian drones from China or Iran, the source said, then marries each one to a grenade or similar explosive device. The result is a nearly invisible, highly precise weapon that allows Hezbollah to carry out targeted – if small-scale – attacks against Israeli forces. China has previously denied supplying weapons to any parties in the conflict and has stressed that it fulfils its international obligations.

While limited in how much damage they can do, the low-cost devices are a potent weapon for Hezbollah.

“Hezbollah already has a fairly sophisticated drone arsenal,” Bendett told CNN. “It has a lot of experienced people with a lot of different experience in UAVs.”

For years, Hezbollah worked with Iran’s financial and technological backing to build a massive rocket and missile arsenal. Before the Gaza war, Israeli officials assessed that Hezbollah had approximately 150,000 rockets, including long-range and precision munitions. But throughout the war, as a result of Israel targeting the arsenal and Hezbollah firing the rockets, Israeli officials have estimated that Hezbollah retains just 10% of its rockets.

Unable to match the Israeli military’s power or technology, the Iran-backed militia has instead employed asymmetric warfare, much as Iran has done against the United States and Israel.

The IDF has countered by using nets and other physical barriers – as seen in Ukraine – to prevent incoming drones from reaching troops, but an Israeli military official acknowledged it’s an imperfect solution to a low-tech problem.

“It’s not foolproof – not as much as we would like,” said the official. The IDF is working with its intelligence directorate to find better ways to counter fiber-optic drones, the official said, but the danger remains.

“It’s a threat that we are still adapting to,” the official said. The problem is magnified when Hezbollah launches multiple drones at once, potentially overwhelming a system that’s not fully up to the task of identifying the incoming drones.

“Hezbollah is learning fast. They are trying to coordinate attacks, so it’s a threat.”