Israel is rationing best interceptors—and Iran’s missiles are getting through

JERUSALEM—Israel has begun rationing its use of high-end missile interceptors, hoping to preserve stocks of its most capable defensive weapons in the face of daily Iranian barrages that haven’t let up through four weeks of war.

A pair of Iranian ballistic missiles recently scored direct hits on the towns of Dimona and Arad after Israel tried and failed to intercept them with modified versions of less advanced munitions.

Israel faced another alarming bombardment Thursday, with residents reporting sirens going off constantly and several missile hits across the country.

Israel has successfully made heavy use of its top of the line Arrow interceptors to shoot down ballistic missiles so far in the war and in the conflict with Iran last June. It recently has been using upgraded versions of its David’s Sling system, which was designed to shoot down rockets and shorter-range ballistic missiles, to intercept bigger and longer-range varieties—with mixed results.

The decision to use less-capable munitions reflects the pressure militaries across the region are under as they burn through expensive, difficult-to-manufacture weapons to fend off attacks from Iran’s mass-produced missiles and drones.

The U.S. and Israel have knocked out much of Iran’s capability to fire missiles but not all of it, turning the war in part into a race to see which side runs out first.

“The number of interceptors of every type is finite,” said Tal Inbar, a senior analyst at the U.S.-based Missile Defense Advocacy Alliance. “As the fighting goes on, it goes down. And as it goes down, you have to make more careful calculations about what to use.”

Iran has fired more than 400 missiles and hundreds of drones at Israel since the start of the war. While the numbers were higher in the early days, the barrages have remained relatively steady in recent weeks, with the additional challenge of Hezbollah firing dozens of projectiles at Israel every day.

With every incoming missile, officials must decide whether to let it fall in unoccupied areas or shoot it down, and if so with what system. They also have to consider preserving stockpiles to cover the range of threats that could arise in the days ahead.

Israel’s multilayered air-defense system, much of which it developed alongside the U.S., uses different munitions designed to confront different types of threats.

On the lower tier is the Iron Dome, which is used to shoot down short-range rockets at a cost of tens of thousands of dollars per interceptor, followed by David’s Sling, which can be used against long-range rockets, tactical ballistic missiles and cruise missiles.

At the upper tier is the Arrow 3, which intercepts long-range ballistic missiles that leave the Earth’s atmosphere and is among the best antimissile munitions in the world. An earlier version known as Arrow 2 is still used for medium to long-range missile threats.

Israel entered the current conflict with stocks of its Arrow interceptors diminished by the war in June.

Air-defense systems project the trajectory of incoming fire based on sensors such as radars and recommend which interceptors operators should use.

Israel has tried to give itself more options with modifications including software updates that make its lower-tier interceptors more capable of handling threats from longer-range missiles.

David’s Sling went through a series of upgrades and tests just before the war with Iran to expand its range.

“We are trying to stretch it to the upper tier and distance the interception from the ground as much as possible,” said Ran Kochav, a brigadier general in the reserves and former commander of Israel’s air and missile defense forces. “It works well in some areas, and in others it doesn’t.”

Iron Dome, which was originally developed to intercept rockets at a range of about 45 miles, also has been upgraded and is now used to shoot down missiles and longer-range rockets and drones.

“Today, it intercepts rockets at a range of hundreds of kilometers as well as UAVs,” Kochav said.

When the Iranian missile hit the southern city of Dimona, home to Israel’s main nuclear facility, and another crashed down in the courtyard of an aging apartment block in the nearby city of Arad, it raised alarm bells across the country.

The community across the street from the Dimona strike has relocated some of its elderly to a large bomb shelter in a school, where they plan to stay until the end of the war.

“This is not over,” said Ahmadiel Ben Yehuda, 69, who lives near the Dimona blast site. “We’re reminded every few hours with warnings on the phone and new sirens and blasts.”

Gulf countries such as the United Arab Emirates, Qatar and Bahrain are under similar pressure and have been asking the U.S. for interceptors. To help address the capacity concerns, the U.S. has flooded the region with counterdrone systems, which can shoot down lower and slower-flying threats like Iran’s Shaheds.

But the world is facing problems with supplies. Among them, interceptors needed for the U.S. Thaad systems, which are used to defend U.S. allies, are running low amid a global shortage in stockpiles and long production cycles. At least one Thaad system deployed to Jordan was damaged in an Iranian drone attack, according to U.S. officials.

“We are vaporizing many years of production in the last couple weeks,” said Tom Karako, director of the Missile Defense Project at the Center for Strategic and International Studies. “Even if we completely maximize production with the forthcoming missions ramp, which we need to, it will still be many years before we replace what was just used.”

Analysts said the model of the fight against Iran isn’t sustainable over the long term. Combatants in other battlefields such as Ukraine are likely to suffer the consequences of the shortages that are now being deepened.

“These are scarce national resources, and we need them for other parts of the world,” Karako said. “That’s not something that we can keep doing.”

Write to Anat Peled at anat.peled@wsj.com