Iran is boasting that it has hypersonic missiles and says it already has begun firing the cutting-edge weapons at Israel.

There is no evidence that Iran has unleashed the missiles, and experts are skeptical of the claim.

But the use of these fast-moving projectiles could test Israel’s vaunted missile-defense system and alter the course of the fighting between the two bitter enemies.

Here’s a closer look at these advanced weapons:

What is a hypersonic missile and what makes them so feared?

Iran’s paramilitary Revolutionary Guard claimed Wednesday that it had fired what it said were hypersonic “Fattah 1” missiles toward Israel. But whether these missiles are hypersonic is a matter of debate.

Simply put, hypersonic weapons are any missile that travels beyond Mach 5, five times the speed of sound. Ballistic missiles, fired high up or outside the earth’s atmosphere, routinely reach this speed.

But in modern warfare, experts say hypersonic weapons must also have advanced navigation systems – making them nimble and capable of changing their trajectory. This can challenge traditional defense systems, said Jack Watling, a senior fellow at the Royal United Services Institute.

Traditional ballistic missiles fly on a trajectory that missile-defense systems like the U.S.-made Patriot can anticipate. Cruise missiles, which can hug terrain, or hypersonic missiles, which are fired to a lower altitude, have less predictable trajectories and are harder to stop.

“Radar can see a missile on a ballistic curve because it’s above the radar horizon. If it’s a hypersonic glide vehicle, it can fly lower and hills get in the way,” Watling said. “That further reduces the time that you will have to engage because if it comes over the horizon, you suddenly see it, and then it’s over.”

Who has hypersonic missiles or is developing them?

Experts say the U.S. and China are the only countries that have developed new-generation hypersonic missiles — but neither have used them in battle. Other nations such as Russia, North Korea and Pakistan have tested or used missiles with similar but less sophisticated technology.

“In the way that it is currently being used, the term ‘hypersonic’ often has little to no meaning and at the same time fuels competitive dynamics and a fear of missing out on the technology,” according to a 2022 report by the Stockholm International Peace Research Institute.

The U.S. says it is putting hypersonic missiles on a stealth destroyer and is developing and testing other programs.

China tested its first hypersonic missile in 2017 and has since developed an array of hypersonic weapons that the U.S. Defense Department says could threaten Hawaii, Alaska and even the continental U.S.

Secretary of Defense Pete Hegseth has warned of China’s “ huge investments ” in military technology including hypersonic weapons.

What are Iran’s capabilities?

Most countries can’t make missiles that would weather the temperature and momentum stresses of these incredibly fast munitions, Watling said.

“This is a hugely complicated task. The Iranians don’t have the capacity to manufacture them,” he said.

Most of the missiles Iran has deployed against Israel travel at hypersonic speed, but are barely maneuverable, so are not considered true hypersonic missiles, said Yehoshua Kalisky, a senior researcher at the Israeli think tank INSS and a former scientist in Israeli defense industries.

The missile it has launched, the Fattah 1 has had minimal success. Israel says Iran has fired over 400 missiles, with over 40 causing damage or casualties.

“Israel is able to intercept more than 95% of the missiles because speed is not crucial,” said Kalisky. “What is important is the maneuverability of the incoming missiles, and so far the maneuverability of these missiles is limited.”

He said Iran has two fast and maneuverable missiles, the Khorramshahr and Fattah 2, that would be “more difficult” to intercept. But neither have been deployed.

When and where have hypersonic missiles been used?

Russia has claimed to have used hypersonic missiles in its war against Ukraine, but experts say that while fast they do not maneuver enough to be considered true hypersonic weapons.

Russia President Vladimir Putin boasted of the development and use of the Orenshik in Ukraine — claiming that flies “like a meteorite” at 10 times the speed of sound, and that it was immune to any missile defense system. Ukrainian military officials said it reached Mach 11.

The Pentagon said last December the Oreshnik fired was an experimental type of intermediate-range ballistic missile.

Russia has also claimed its Kinzhal missile is hypersonic, but Ukraine has been able to intercept them with the U.S.-made Patriot missile defense system.

During the recent fighting between India and Pakistan over the disputed territory of Kashmir, Pakistan said it destroyed a Russian-built S-400 air defense system in India’s border Punjab state with hypersonic missiles launched from a warplane.

Brazil, Australia, the United Kingdom, France, Germany, Iran, Japan, South Korea and North Korea all have hypersonic weapons programs. The European Union is studying how to develop an interceptor for hypersonic missiles as it ramps up defense spending to counter the Russian threat.