Russian President Vladimir Putin warned the West on Thursday that letting Ukraine use longer-range weapons to strike targets inside his country would put NATO "at war" with Russia.
The stark warning came as US and UK officials discussed Kyiv's demand for them to ease rules on firing Western weapons into Russia, more than two and a half years into Moscow's invasion of Ukraine.
Moscow meanwhile also claimed to have recaptured a swath of territory in its western Kursk region, where Ukrainian forces had made advances in recent weeks.
Putin spoke after US Secretary of State Antony Blinken promised that Washington would quickly review Kyiv's long-standing request for more leeway to use Western-supplied weapons to strike Russian targets.
"This would in a significant way change the very nature of the conflict," Putin told a state television reporter.
"It would mean that NATO countries, the US, European countries, are at war with Russia," he said.
"If that's the case, then taking into account the change of nature of the conflict, we will take the appropriate decisions based on the threats that we will face."
Speaking at a press conference in Warsaw on Thursday, Blinken said the United States would "adjust, we'll adapt as necessary, including with regard to the means that are at Ukraine's disposal to effectively defend against the Russian aggression".
- Kursk counteroffensive -
Kyiv launched a shock incursion into Russia's Kursk border region on August 6, advancing kilometres into Russian territory and seizing dozens of settlements.
But Ukrainian President Volodymyr Zelensky acknowledged that Moscow's troops were striking back.
Ukraine's attack -- the biggest by a foreign army on Russian territory since World War II -- caught Moscow off guard as its invasion drags through its third year.
Russia has insisted from the start that it would expel Ukrainian troops from the region.
But until now it had appeared to be on the back foot, marshalling a humanitarian response and evacuating around 150,000 people.
"Units of the 'North' group of troops liberated 10 settlements within two days," Russia's defence ministry said on Telegram.
Speaking at a press conference in Kyiv, Zelensky said "Russians have launched counteroffensive actions".
He did not provide details on the extent of Russian operations, but said the incursion was still "going in line with our Ukrainian plan".
Ukraine's commander-in-chief of armed forces Oleksandr Syrsky said in late August that Kyiv had seized 100 settlements and almost 1,300 square kilometres (500 square miles) of Russian territory.
Kyiv says it does not want to annex parts of Russia -- as Moscow has done in eastern Ukraine -- but hopes it can force its enemy to divert troops and make the territory gains a bargaining chip in any negotiations.
- Longer-range strikes -
Kyiv wants more flexibility to hit Russian airfields and other military targets further from the front lines that it says are crucial to Moscow's invasion.
Washington currently authorises Ukraine to only hit Russian targets in the occupied parts of Ukraine and some in Russian border regions directly related to Moscow's combat operations.
"The delay in this process leads to Russia moving these military targets deeper into Russia," Zelensky said.
The Kremlin said Wednesday that it would respond "appropriately" if the restrictions were lifted.
President Joe Biden is expected to review the Ukrainian requests Friday in a White House meeting with British Prime Minister Keir Starmer.
Biden has been hesitant to take actions that risk triggering direct conflict between nuclear-armed Russia and the United States.
Speaking in Poland after a trip to Kyiv, Blinken was also set to discuss an appeal by Ukrainian Foreign Minister Andriy Sybiga for neighbouring countries to help shoot down missiles over its western region.
Russia has increased its air attacks on Ukraine in recent weeks amid the Kursk offensive, including targeting the country's energy infrastructure as winter looms.
- 'Another war crime' -
Moscow's forces are also ramping up the pressure on the battlefield in the eastern Donetsk region, where the most intense fighting is taking place.
The International Committee of the Red Cross and Ukrainian officials said Thursday that three Ukrainian aid workers were killed by shelling in eastern Ukraine.
Zelensky said ICRC vehicles were hit in the Donetsk region, calling it "another Russian war crime".
The city administration in the key eastern logistics hub of Pokrovsk said Russia had cut off the water supply to the city, around 10 kilometres (six miles) from the front line.
Zelensky also said an Egypt-bound cargo ship carrying wheat was hit by a Russian missile in the Black Sea, shortly after leaving Ukrainian waters.
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