The Kremlin has directly blamed the United States for an attack on Crimea with US-supplied ATACMS missiles that killed at least four people and injured 151, and Moscow has formally warned the US ambassador that retaliation will follow.
The war in Ukraine has triggered the biggest confrontation between Russia and the West since the 1962 Cuban Missile Crisis, and Russian officials have said that the conflict is entering the most dangerous escalatory phase to date.
But directly blaming the United States for a deadly attack on Crimea - which Russia annexed in 2014 and now considers to be Russian territory although most of the world considers it to be 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. Just ask them this question," Kremlin spokesman Dmitry Peskov told reporters on Monday.
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.
Russia said that the United States had supplied the weapons, while US military specialists had aimed the weapons and provided data for them.
Neither Ukraine nor the United States has commented on the attack.
Russia summoned US ambassador Lynne Tracy to the foreign ministry where she faced accusations that Washington was "waging a hybrid war against Russia and has actually become a party to the conflict".
The attack, Russia told Tracy, would "not go unpunished. Retaliatory measures will definitely follow."
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 that Russia does not want a conflict with the US-led NATO alliance.
US President Joe Biden has repeatedly ruled out sending US troops to fight in Ukraine and said shortly after the 2022 Russian invasion that a direct confrontation between NATO and Russia would mean World War III.
Putin presents the Ukraine war as part of a much broader struggle with the US, which he says ignored Moscow's interests after the Soviet Union's 1991 break-up and then plotted to cleave Russia apart and seize its natural resources.
Leaders of the West and Ukraine have cast the war in Ukraine as an imperial-style land-grab.
The West denies that it wants to destroy Russia, which in turn denies that it intends to invade any NATO member state.
Since the United States allowed Ukraine to use some US weapons against Russia, and Britain suggested Kyiv could do the same with British weapons, the Kremlin has sent several signals that it views the moves as a serious escalation.
Putin ordered drills to practise the deployment of tactical nuclear weapons, suggested Russia could deploy conventional missiles in striking distance of the United States and its allies, and sealed a mutual defence pact with North Korea.
Washington still prohibits Kyiv from striking Russia with ATACMS, which have a range of up to 300km, and other long-range US-supplied weapons.
Putin said on Thursday that Russia might supply weapons to North Korea in what he suggested would be a mirror response to the Western arming of Ukraine.
Asked what the Russian response would be to the attack in Crimea, Peskov recalled Putin's words on June 6 about supplying conventional weapons to regions near to the US and its allies.
"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."
with AP
