Russia has said the West is playing with fire by considering allowing Ukraine to strike deep into Russia with Western missiles, and has cautioned the United States that World War III would not be confined to Europe.
Ukraine attacked Russia's western Kursk region on 6 August and has carved out a slice of territory in the biggest foreign attack on Russia since World War II.
President Vladimir Putin said there would be a worthy response from Russia to the attack.
Sergei Lavrov, who has served as Putin's foreign minister for more than 20 years, said that the West was seeking to escalate the Ukraine war and was "asking for trouble" by considering Ukrainian requests to loosen curbs on using foreign-supplied weapons.
Since invading Ukraine in 2022, Mr Putin has repeatedly warned of the risk of a much broader war involving the world's biggest nuclear powers, though he has said Russia does not want a conflict with the US-led NATO alliance.
"We are now confirming once again that playing with fire - and they are like small children playing with matches - is a very dangerous thing for grown-up uncles and aunts who are entrusted with nuclear weapons in one or another Western country," Mr Lavrov told reporters in Moscow.
"Americans unequivocally associate conversations about Third World War as something that, God forbid, if it happens, will affect Europe exclusively," Mr Lavrov said.
Mr Lavrov added that Russia was "clarifying" its nuclear doctrine.
Russia's 2020 nuclear doctrine sets out when its president would consider using a nuclear weapon: broadly as a response to an attack using nuclear or other weapons of mass destruction or conventional weapons "when the very existence of the state is put under threat".
Ukrainian President Volodymyr Zelensky said earlier this month that the assault on Russia's Kursk region showed that Kremlin threats of retaliation were a bluff.
Mr Zelensky said Ukraine, because of the restrictions imposed by allies, could not use the weapons at its disposal to hit some Russian military targets. He urged allies to be bolder in their decisions about how to help Kyiv in the war.
Russia has said that Western weaponry, including British tanks and US rocket systems, have been used by Ukraine in Kursk. Kyiv has confirmed using US HIMARS missiles to take out bridges in Kursk.
The US has said it was not informed about Ukraine's plans ahead of the surprise incursion into Kursk. The US has also said it did not take any part in the operation.
Mr Putin's foreign intelligence chief, Sergei Naryshkin, said earlier that Moscow did not believe Western assertions that it had nothing to do with the Kursk attack. Russian Deputy Foreign Minister Sergei Ryabkov said the involvement of the United States was "an obvious fact".
The New York Times reported that the United States and Britain provided Ukraine with satellite imagery and other information about the Kursk region in the days after the Ukrainian attack.
The Times said that the intelligence was aimed at helping Ukraine keep better track of Russian reinforcements.
Six killed in second day of major Russian attacks on Ukraine
Russia launched missile and drone attacks targeting scores of Ukrainian regions and killing at least six people, officials said, a day after Russia's biggest air attack of the war on its neighbour.
Three people were killed when a hotel was "wiped out" by a missile in the central city of Kryvyi Rih, regional officials said.
Five people were injured, and one person was still missing after the strike, Serhiy Lysak, governor of the Dnipropetrovsk region that includes Kryvih Rih, said on Telegram.
Separately, three people were killed in drone attacks on the southeastern Ukrainian city of Zaporizhzhia.
Three people were also injured in the Zaporizhzhia region and four were hurt in a missile strike on the northeastern region of Kharkiv overnight, local authorities said.
President Volodymyr Zelenskiy said that Ukraine would retaliate against Russia for its attacks.
He asked allies to consider joint air defence operations and provide long-range capabilities after Russia pummelled Ukrainian energy infrastructure with more than 200 missiles and drones on Monday.
During today's attack, Ukraine downed five out of 10 incoming missiles and 60 out of 81 drones, the air force said.
The Ukrainian air force lost track of 10 more drones that have likely come down somewhere on its territory, it said.
One more crossed into Belarusian territory.
The Russian defence ministry said its forces had carried out a high-precision weapon strike on Ukraine overnight, the Interfax news agency reported.
Russia denies targeting civilians since launching a full-scale invasion of Ukraine in February 2022, although thousands have been killed.
Several Russian military bloggers said Moscow's attacks were an "act of retaliation" for Ukraine's surprise incursion into Russia's western Kursk region - the first such action since World War II.
Kyiv test fires first Ukraine-made ballistic missile, says Zelensky
Mr Zelensky has said that his military has recently carried out the first successful test of a domestically-produced ballistic missile.
His announcement in Kyiv came in the wake of two consecutive nights of large-scale Russian bombardments across Ukraine that killed several people and battered energy infrastructure.
"There has been a positive test of the first Ukrainian ballistic missile. I congratulate our defence industry on this. I can't share any more details about this missile," he said at a press conference in the Ukrainian capital.
Ukraine has been attempting to develop its own arms industry and encourage military production on its own territory as part of efforts to become less dependent on Western military aid.
Mr Zelensky said last week that his forces had deployed in combat for the first time a Ukrainian-made long-range "rocket drone" called Palianytsia.
Kyiv has signed agreements with Western arms companies on the production of small arms and ammunition.
Mr Zelensky also said that his military had deployed Western-supplied F-16 fighter jets to counter Russia's recent large-scale drone and missile attacks.
"We destroyed already some missiles and drones using the F-16," he said.
He added that the war with Russia would eventually end through dialogue, but that Kyiv had to be in a powerful position at a summit that it hopes to convene this year to advance its vision of peace.
It comes as Ukraine's top commander said Ukrainian forces were still advancing in Russia's Kursk region, but warned that Moscow was building up its forces on the eastern Pokrovsk front, where Russian troops have been advancing.
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General Oleksandr Syrskyi said Ukraine's three-week-old thrust into Russia's Kursk region has captured 100 settlements.
Moscow's troops were trying to counterattack in the area and encircle Kyiv's forces, but those attempts were being repelled, he added.
He said that one of the objectives of the Kursk operation was to divert Russian forces from other areas, primarily the Pokrovsk and Kurakhove sectors.
"The Kursk operation diverted a significant number of its forces," he said, noting that Russian troops had been drawn from Ukraine's south.
"As of now, we can say that around 30,000 servicemen have been sent to the Kursk front and this figure is growing."
But he said Russia was strengthening its force on the Pokrovsk front.
Disclosing a number for the first time, General Syrskyi said that Ukraine had captured 594 Russian servicemen during its operation in the Kursk region.
Additional reporting AFP
