“We have, of course, informed all the affected countries that we are taking this extraordinary step, and we’re making every effort to minimize any negative impact,” Kirby said. He added that when U.S. allies were told their shipments would be delayed, “the response we got was broadly supportive … because they know how serious the need is in Ukraine.”

It’s the latest in a series of recent steps by the Biden administration to reinforce Ukraine as it defends against an aggressive push by Moscow to break the country’s morale. Throughout the spring, the White House has approved large weapons transfers to replenish depleted stocks, rescinded its strict prohibition on the use of U.S. arms for strikes inside Russia, and solidified a 10-year security pact with Kyiv while leaders of the Group of Seven major democracies said they would tap billions of dollars in frozen Russian assets to sustain Ukraine’s fight.

For months, the government of President Volodymyr Zelensky has pleaded with its supporters for substantial increases to its air defense inventory as Russia bombards Ukraine with missiles, drones and glide bombs. To date, those requests have yielded one additional Patriot system from the United States and another from Romania.

The Patriot and NASAMS systems are the two most sophisticated air defense platforms the West has provided to Ukraine. The Patriot, valued at $1 billion, is especially coveted by Kyiv. It is the only system in its arsenal that has proved capable of shooting down Russian hypersonic missiles, which are especially difficult to detect and defend against.

Ukraine has struggled mightily to protect against Russian glide bombs, however, because they are nearly impossible to bring down once launched. The solution, Ukrainian officials say, is to target the aircraft that fire those weapons, but to do so in the near term would require moving their limited number of Patriots closer to Russia’s border — making them more vulnerable to attack.

Longer term, Ukraine hopes its fleet of advanced F-16 fighter jets will prove a formidable counterpunch to the Kremlin’s glide bombs, but the arrival of those aircraft, pledged by Western nations months ago, remains many weeks away, officials say.

Kirby did not specify how long U.S. allies would need to wait for their delayed orders, but said this reprioritization would not impact Taiwan, which faces a threat from China, or Israel, which has endured attacks from Iranian-backed groups in the Middle East, including Hezbollah and Houthi militants.

A State Department official declined to say which U.S. partners may be affected or when the munitions would be redirected, citing ongoing diplomatic conversations about the process. Kirby indicated the interceptors bound for Ukraine would include missiles “rolling off the production line.”

The proposal was floated by President Biden’s national security adviser, Jake Sullivan, in early April, as Republican lawmakers delayed approval of a major national security spending bill to provide more weapons and assistance to Ukraine, a senior administration official said. Sullivan’s plan coalesced in the ensuing weeks, and Biden told Zelensky at last week’s G-7 gathering in Italy that the United States would shuffle its air defense exports to prioritize Ukraine, this official said, speaking on the condition of anonymity to discuss internal deliberations.

The policy change was first reported by the Financial Times.

While the Patriot has been used primarily to defend against Russian missile attacks, Ukrainian air defenders have used them to bring down enemy aircraft, too.

Ukraine has utilized Patriot systems in a “historic” way by bringing them close to the front line, stretching the limits of their capabilities, U.S. Army Col. Rosanna Clemente, the assistant chief of staff of the 10th Army Air and Missile Defense Command, said at a recent symposium.

The so-called “SAMbush” — short for surface-to-air-missile ambush — brought down a Russian A-50 command and control aircraft in January, Clemente said. Ukraine reported that it shot down that aircraft over the Sea of Azov.

Washington late last month allowed Ukraine to use U.S.-provided weaponry against limited military targets inside Russia from which its forces are attacking or preparing to attack.

Kyiv has been allowed to “use air defense systems supplied by the United States to take Russian planes out of the sky, even if those Russian planes are in Russian airspace, if they’re about to fire into Ukrainian airspace,” said Maj. Charlie Dietz, a Pentagon spokesperson.

That separate policy has been in effect for more than a year, the senior administration official said previously, noting that Ukraine brought down several helicopters and fighter jets using Patriots. “There’s never been a restriction” on using U.S. air defenses to shoot down incoming Russian aircraft in Russian territory, the official said.

Ellen Nakashima contributed to this report.