Russian Foreign Ministry summoned the American ambassador on Monday to protest what it says was the use of U.S.-made advanced missiles in a Ukrainian attack on Russian-annexed Crimea that reportedly killed four people and wounded more than 150.

Washington “has effectively become a party” to the war on Ukraine’s side, the ministry said in a statement, adding, “Retaliatory measures will certainly follow.” It did not elaborate.

There was no immediate comment from U.S. or Ukrainian officials. The Associated Press could not independently verify Russia’s claims about the missiles used.

Kyiv’s forces have relied heavily on Western-supplied weaponry since Russia’s invasion more than three years ago. The military aid has been crucial in allowing Ukraine to hold the Kremlin’s army at bay, with few major changes along the 1,000-kilometer (620-mile) front line in eastern and southern Ukraine for many months.

Some Western countries have hesitated over providing more — and more sophisticated — help for Kyiv’s army because of concerns about potentially provoking the Kremlin. But as Ukraine has at times struggled to hold the line against Russia’s bigger and better-equipped military, Western leaders have gradually relented and granted more support.

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US Missiles in Ukraine

In the latest key development, the Pentagon said last week that Ukraine’s military is being allowed to use longer-range missiles provided by the U.S. to strike targets inside Russia if it is acting in self-defense. Since the outset of the war, the U.S. had maintained a policy of not allowing Ukraine to use the weapons it provided to hit targets on Russian soil for fear of further escalating the conflict.

Crimea, which Russian annexed from Ukraine in 2014 in a move that most of the world rejected as unlawful, long had been declared a fair target for Ukraine by its Western allies.

Russian authorities said that the dead in Sunday’s attack included two children who were hit by falling debris from Ukrainian missiles that were shot down over a coastal area in Sevastopol, a port city in Crimea. It said cluster munitions, which critics say harm more civilians than combatants, were also used.

Russia said the missiles were U.S.-made ATACMS, a long-range, guided missile. It summoned U.S. Ambassador Lynne Tracy to the Foreign Ministry.

The targeting and “mission input” for such missile attacks is carried out by U.S. military experts, the ministry statement alleged, saying the United States bears “equal responsibility for this outrage” with the Ukrainian authorities.

It went on to say that “allowing strikes deep inside the Russian territory will not be left unanswered.”

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Russian Retaliation

Meanwhile, the Russian Defense Ministry on Monday reported striking a “major logistical hub” of the Ukrainian military that held Western-supplied missiles and other weapons.

It said the strike was carried out by warplanes, drones, ground-launched missiles and artillery. The ministry didn’t name its location.