Ukraine’s President Volodymyr Zelensky expressed frustration after the U.S. and its allies swooped in to defend Israel against a massive Iranian attack over the weekend, highlighting the limits of Western support for Kyiv.

Ukrainian cities have been under fire for more than two years from Russian missiles and explosive-laden drones of the same type used in Tehran’s attack on Israel.

The U.S. and a coalition of Western and Arab partners shot down nearly all the 170 drones, around 120 ballistic missiles and about 30 cruise missiles fired by Iran on Saturday before they even reached Israeli airspace.

Ukrainians, in contrast, have been pleading for months for greater protection against Russian attacks that have intensified while vital  aid from the U.S. remains stalled in Congress .

“Now the whole world has seen from the actions of Israel’s allies in the sky and neighboring countries how effective unity can be in the protection from terror,” Zelensky said. “Terror must lose everywhere and completely—not somewhere more and somewhere less.”

Ukrainian officials have refrained from directly criticizing U.S. policy publicly, wary of being seen as ungrateful for Washington’s support. Others have been less diplomatic.

“There’s nothing but American timidity that explains why we don’t do that for Ukraine,” said John Herbst, a former U.S. ambassador to Ukraine and retired career diplomat.

Herbst noted that U.S., British and French forces defending Israel on Saturday only intercepted Iranian missiles and drones, avoiding any engagement with Iranian forces that might lead to a direct military conflict.

Israel’s Iron Dome anti-missile system intercepts rockets launched from Lebanon towards Israel over the Israeli Lebanese border, as seen from northern Israel, April 12, 2024. REUTERS/Ayal Margolin ISRAEL OUT. NO COMMERCIAL OR EDITORIAL SALES IN ISRAEL

Administration officials have said that their approach to Ukraine and Russia is appropriate, given the risk of escalation.

The U.S. and its allies in the North Atlantic Treaty Organization in the early days of the war resisted Ukrainian appeals for a no-fly zone, citing logistical challenges and the risk of coming into direct conflict with the Russian military.

Since then, fears of escalation have also kept the U.S. and allies from supplying Ukraine with other weapons Kyiv says it needs to fight its larger neighbor, including longer-range missiles.

Recent delays in aid from the U.S. have emboldened Russia, which is clawing territory from outgunned and exhausted Ukrainian troops in the east of the country. The U.S. has meanwhile chastised Ukraine for using long-range drones of its own design to strike targets deep inside Russian territory.

Rather than helping Ukraine create the kind of air-defense network Israel has, the West has provided Kyiv with a patchwork of equipment that blunted Russia’s attacks for many months. But the country’s stockpiles of air-defense interceptors are being depleted by an intensifying campaign of Russian strikes targeting power plants and other civilian infrastructure.

The mayor of Ukraine’s second-largest city, Kharkiv, which has borne the brunt of the latest strike campaign, said he could only wish for Israel-style protection.

“Of course I want such an opportunity to defend our city against this kind of attacks,” said Mayor Ihor Terekhov said. “It is necessary to protect them both,” he said of Israel and Ukraine.

“People are beginning to sour on the U.S.,” said Volodymyr Dubovyk, associate professor and director of the Center for International Studies at Odesa I.I. Mechnikov National University.

The U.S. was “a decisive factor for Ukraine in the first two years of the war, but now of course there’s a huge slowdown,” Dubovyk said.

While both Ukraine and Israel have faced air attacks from their enemies, the two countries have fundamentally different security relationships with the U.S.

Few countries have closer defense ties with the U.S. than Israel. The two share highly classified intelligence and cooperate extensively on regional policies. Despite frequent disputes and widespread frustration in Washington with Israeli Prime Minister Benjamin Netanyahu , who often ignores requests and pressure from the White House, Israel is one of America’s closest allies.

The U.S. doesn’t have a defense treaty with Israel, binding it to provide aid, but Israel for decades has enjoyed a special relationship with the U.S. as America’s closest partner in the Middle East. Israel is the largest cumulative recipient of U.S. foreign assistance since World War II, according to the Congressional Research Service.

Under a 10-year accord struck in 2019, Washington committed to provide Israel with $38 billion in military aid through 2028. The two sides cooperate closely on advanced military systems including the Iron Dome air-defense network that Israel employed on Saturday to block the Iranian attack.

Firefighters work at the site where residential buildings were damaged by a Russian missile strike, amid Russia’s attack on Ukraine, in Odesa, Ukraine April 20, 2024. REUTERS/Serhii Smolientsev

Israel’s relations with Washington are cemented by exceptionally effective lobbying in the U.S. The longstanding campaign brings together Jews with direct links to Israel and non-Jews who either support the country’s right to exist or see it as a bulwark against common adversaries, such as Iran and Syria. Even religious support for Israel extends beyond Jews who see the country as their homeland to Christians who want to maintain Israel’s control over holy sites.

Since Russia’s full-scale invasion of Ukraine more than two years ago, some of Kyiv’s backers have proposed an Israeli-style model of Western defense relations, but the concept hasn’t advanced—in part because it would entail a degree of commitment that no major Western power has yet to show.

The Biden administration’s policy toward Ukraine over the past two years has been shaped by concerns that Russian President Vladimir Putin could resort to nuclear arms or other weapons of mass destruction if the U.S. or other NATO members give Kyiv the means to inflict severe damage on Russian forces or the country itself. Putin has suggested he might resort to nuclear weapons in the war.

Iran has advanced its nuclear program so that it could produce enough nuclear fuel for a bomb in days, but U.S. officials say it isn’t actively working on building a nuclear weapon. Security analysts think it is far from being able to use one against Israel or a U.S.-allied target. Russia, in contrast, has the world’s largest store of nuclear weapons.

“Biden has been spooked by Putin’s constant nuclear threats,” said Herbst, the former U.S. ambassador. “We behave as if we’re not a nuclear superpower too.”

Concerns about Russia’s response have prompted the U.S., Germany and some other NATO members to spend weeks or months deciding whether to give lethal systems such as U.S.-made Himars mobile rocket launchers, ATACMS rockets and F-16 jet fighters. The administration ultimately acquiesced on all, but critics say the lengthy decision-making reduced their effectiveness by giving Russia time to prepare. And there are still some systems that Ukraine wants but hasn’t received, such as German Taurus air-launched cruise missiles.

Air-defense equipment, such as American-made Patriot missile batteries, are among the systems that Ukraine received after extensive lobbying but now needs in greater numbers. The U.S., Germany and the Netherlands sent Ukraine its first Patriot surface-to-air interceptors one year ago, and the advanced interceptors have proven highly effective at shooting down Russian rockets, missiles and aircraft.

While Ukraine has received several of the systems, as well as other, less-advanced European and Soviet-designed systems, it lacks enough launchers and projectiles to thwart Russia’s increasingly ferocious air attacks.

European Union foreign-policy chief Josep Borrell said last week that Ukrainian Foreign Minister Dmytro Kuleba had asked for seven Patriot batteries. Borrell said Western militaries have roughly 100 Patriot batteries. “And still we are not able to provide the seven they are asking desperately for,” he said in exasperation.

Zelensky recently said Ukraine would need 25 Patriot air-defense systems to shield itself fully against Russian attacks. Each system would require scores of interceptor missiles.

“We are still grateful for everything the U.S. provided,” said Kharkiv resident Serhiy Zaitsev. “But there is a bit of sadness, because if help had been given to us on time the situation on the front would be completely different and we would have been able to save many more lives.”

Write to Isabel Coles at isabel.coles@wsj.com and Daniel Michaels at Dan.Michaels@wsj.com