Moscow has warned Kyiv and the West against using long-range weapons to attack Russian territory, threatening them with uncontrolled escalation of war.
The warnings on Saturday came after Ukrainian President Volodymyr Zelenskyy said Friday that his plan to achieve victory depended on Washington's decision, a clear reference to the authorization for long-range strikes that Kyiv has long sought from NATO allies.
Andriy Yermak, head of Zelenskyy's office, said on the Telegram messaging app on Saturday: "Strong decisions are needed. Terror can be stopped by destroying the military facilities where it originates."
Kyiv has said such strikes are critical for its efforts to restrict Moscow's ability to attack Ukraine but allies have so far been reluctant to permit them, citing fears Moscow will treat them as an escalation and doubting their efficiency.
While no official decision on the matter has been announced so far, Russian Deputy Foreign Minister Sergei Ryabkov alleged it had already been made and communicated to Kyiv, and that Moscow would have to respond with actions of its own.
"The decision has been made, the carte blanche and all indulgences have been given (to Kyiv), so we are ready for everything," the RIA news agency quoted Ryabkov as saying.
"And we will react in a way that will not be pretty."
'Melted spot'
Former Russian President Dmitry Medvedev, who now serves as deputy chairman of the country's security council, said the West was testing Russia's patience but it was not limitless.
Medvedev said Ukraine's incursion into Russia's Kursk region, which Zelenskyy described as a successful operation slowing Russia's advance, already gave Russia formal grounds to use its nuclear arsenal.
He said that Moscow could either resort to nuclear weapons in the end, or use some of its non-nuclear but still deadly novel weapons for a large-scale attack.
"And that would be it. A giant, grey, melted spot instead of 'the mother of Russian cities,'" he wrote on the Telegram messaging app, referring to Kyiv.
Ukraine's Yermak, in turn, said of Russian President Vladimir Putin: "Loud threats of Putin's regime testify only to his fear that terror may come to an end."
More deaths across Ukraine
Meanwhile, Russian shelling killed at least seven people in four attacks on the south, southeast and east of Ukraine on Saturday, regional Ukrainian governors said.
In the Zaporizhzhia region in southeast Ukraine, Gov. Ivan Fedorov said Russian shells struck an agricultural enterprise in the town of Huliaipole, killing three people.
"All the dead are employees of the enterprise," Fedorov said in a post on the Telegram messaging app. Reuters could not verify details of these latest attacks in the war in Ukraine.
A missile attack in the suburbs of the Black Sea port city of Odessa killed a man and a woman born in 1958 and 1962 and injured a 65-year-old woman, Oleh Kiper, the Odesa regional governor, said in a post on the Telegram messaging app.
"A married couple died," he said, adding that they were found during checks of residential and commercial buildings damaged earlier in the day and that Russian forces had used a prohibited cluster warhead.
Shelling killed a sixth person in the southern region of Kherson, governor Oleksandr Prokudin, said. "A 60-year-old man who suffered serious injuries this afternoon died in hospital," Prokudin wrote on Telegram.
In Kharkiv region, Russia struck the village of Pisky-Radkivski with the high-speed Tornado-S multiple rocket launch system, regional governor Oleh Syniehubov said on Telegram.
The body of a 72-year-old woman was retrieved from the rubble, and two civilians, a man and a woman, were taken to hospital, he added.