Russia on Monday squarely blamed the US for an attack on Crimea with United States-supplied ATACMS missiles that killed at least four people and injured 151, and the Kremlin formally warned the Washington's ambassador retaliation would follow.
The war in Ukraine has deepened a crisis in relations between Moscow and the West, and Russian officials have said the conflict is entering the most dangerous escalation to date.
But directly blaming the US for an attack on Crimea - which Russia unilaterally annexed in 2014 although most of the world considers it part of Ukraine - is a step further.
"You should ask my colleagues in Europe, and above all in Washington, the press secretaries, why their governments are killing Russian children," Kremlin spokesperson Dmitry Peskov told reporters.
At least two children were killed in the attack on Sevastopol on Sunday, according to Russian officials. People were shown running from a beach near Sevastopol and some of the injured being carried off on sun loungers.
Kyiv did not comment on the attack but denies targeting civilians.
Russia said the US had supplied the weapons, while the United States military had aimed them and provided data.
Russia's Foreign Ministry summoned US Ambassador Lynne Tracy and told her Washington was "waging a hybrid war against Russia and has actually become a party to the conflict".
"Retaliatory measures will definitely follow," the ministry said.
Tracy said Washington regretted any loss to civilian life, State Department spokesperson Matthew Miller told reporters, adding Washington provided weapons to Ukraine so it could defend its sovereign territory, including Crimea.
Pentagon spokesperson Major Charlie Dietz said Ukraine "makes its own targeting decisions and conducts its own military operations".
A US official later said Ukraine was not targeting civilians. It seemed the Russians were able to intercept an ATACMS missile that was targeting a missile launcher and the ATACMS exploded with shrapnel raining down on the beach, the US official added.
Russian President Vladimir Putin has repeatedly warned of the risk of a much broader war involving the world's biggest nuclear powers, though he has said Moscow does not want a conflict with the US-led NATO alliance.
Russian retaliatory measures
US President Joe Biden has ruled out sending United States troops to fight in Ukraine and said shortly after the 2022 full-scale Russian invasion a direct confrontation between NATO and Russia would mean World War III.
Putin presents the Ukraine war as part of a wider struggle with the US, which he said ignored Moscow's interests after the Soviet Union's 1991 break-up, then plotted to split Russia apart and seize its natural resources.
Leaders of the West and Ukraine have cast the war as an imperial-style land-grab. The West denies wanting to destroy Russia, which in turn denies intending to invade any NATO member state.
After the US allowed Ukraine to direct some United States weapons into Russia, the Kremlin sent signals it viewed this as a serious escalation.
Putin has ordered drills to practice deployment of tactical nuclear weapons, suggested Russia could station conventional missiles within striking distance of the US and its allies, and sealed a mutual defence pact with North Korea.
Washington still prohibits Kyiv from striking recognised Russian territory with ATACMS, which have a range of up to 300km, and other long-range US weapons.
Putin said on Thursday Russia might supply weapons to North Korea in what he suggested would be a like-for-like response to the Western arming of Ukraine.
Asked what the Russian answer would be to the attack in Crimea, Peskov recalled Putin's words on June 6 about wider placement of conventional weapons.
"Of course, the involvement of the United States in the fighting, as a result of which peaceful Russians are dying, cannot but have consequences," Peskov said.
"Which ones exactly - time will tell."
