The Russian government said the United States bears responsibility for a Ukrainian attack on the Russian-annexed Crimean peninsula that left four people dead and more than 150 people injured and promised there would be "repercussions."
"We've seen an absolutely barbaric missile attack in Crimea," Kremlin spokesman Dmitry Peskov said, according to the TASS news agency. "We understand perfectly well who is behind it, who targets these technically sophisticated missiles. It is not the Ukrainians who provide for these launches."
"Naturally, the US’ direct involvement in combat which resulted in Russian civilians dying cannot go without any repercussions. Time will tell which ones precisely."
The Russian Foreign Ministry also summoned the US ambassador to Moscow, Lynne Tracy, following the attack on the Crimean city of Sevastopol on Sunday.
The Kremlin says Washington not only supplies arms to Kiev, but that US specialists also help oversee the flight path of the advanced missiles.
The Russian Defence Ministry said Ukraine fired five US-supplied ATACMS missiles, four of which were intercepted.
On Monday, authorities in Sevastopol declared a state of emergency.
Russian news agencies published a decree issued by Governor Mikhail Razvoshayev that read: "I decree ... to declare a state of emergency on the territory of the city of Sevastopol until further order."
The state of emergency may restrict certain constitutional rights, such as freedom of movement, to ensure public safety.
Most of the victims were sunbathing on a city beach when rocket debris fell and exploded, with no air raid warning preceding the incident.
A Ukrainian government official said on Monday that the victims of the Crimean missile strike were "civilian occupiers."
Mykhailo Podolyak, a political advisor in the president's office, wrote on Telegram: "There are no 'beaches', 'tourist zones' and other fictitious signs of 'peaceful life' in Crimea and cannot be."
Russia currently occupies around a fifth of Ukraine's territory, including the southern Crimean peninsula, which it illegally annexed in 2014.
It is also a popular beach destination. In response to the incident, the affected beach area located near the Sevastopol naval base has been closed for swimming.
The main harbour of Russia's Black Sea fleet and the Belbek military airfield are both located near Sevastopol.
Since Russia's full-scale invasion began two years ago, Moscow has been using Crimea as a supply and attack base against Ukraine. The area has also become a target of attacks by the Ukrainian military.