President Joe Biden has said his administration is “working out” whether to authorise Ukraine to use United States-made, long-range weapons inside Russia, provoking anger in Moscow.

Ukrainian President Volodymyr Zelenskyy has repeatedly called for restrictions on Western-supplied, long-range weapons to be lifted so his forces can strike airfields, ammunition depots, and command centres deep inside Russia, also increasing the costs of the invasion for Moscow.

Biden, when asked by reporters on Tuesday whether he would allow Ukraine to use such missiles to target sites inside Russia, said: “We’re working that out right now.”

His comments followed a US assessment that Iran has been supplying missiles to Russia.

The remarks drew swift condemnation in Moscow, with Vyacheslav Volodin, the head of State Duma, the lower house of the country’s parliament, saying such a move would make the US and its allies a party to the war and prompt Russia to deploy more powerful weapons.

Allowing Ukraine to use US-made missiles capable of striking deep within Russia would be a major policy shift for Washington. Despite sending more than $56bn in military support to Kyiv, the US has so far been wary of escalating tensions with long-range missiles.

Ukraine’s other allies have supplied weapons with restrictions on how and when they can be used inside Russia, fearing that Ukrainian strikes could prompt Russian retaliation that draws NATO countries into the war.

“We need to have this long-range capability, not only on the divided territory of Ukraine but also on the Russian territory, so that Russia is motivated to seek peace,” Zelenskyy said last week in Germany.

“We need to make Russian cities and even Russian soldiers think about what they need: peace or Putin,” he said, referring to Russian President Vladimir Putin.

‘Can Ukraine effectively use it’?

Questions over Ukraine’s use of long-range weapons are expected to be on the agenda as US Secretary of State Antony Blinken and British Foreign Secretary David Lammy visit Ukrainian capital Kyiv on Wednesday.

Blinken, speaking to reporters in London before the trip, declined to say whether Washington would greenlight such weapons for use in Russia, saying numerous factors were under consideration.

“It’s not just the system itself that counts,” said Blinken. “You have to ask: Can the Ukrainians effectively use it, and sometimes that requires significant training, which we’ve done. Do they have the ability to maintain it?”

On Wednesday, Lammy said Britain had sanctioned 10 ships in Russia’s “shadow fleet” of vessels which it says use illicit practices to avoid Western embargoes on Russian oil.

“Today’s sanctions further undermine Russia’s ability to trade in oil via its shadow fleet,” he said in a statement. “Alongside our partners, we will continue to send a stark message to Russia that the international community stands with Ukraine and we will not tolerate this illicit fleet.”

The US’s potential shift on Ukraine’s long-range missile usage comes as Moscow’s forces keep advancing in the Donbas region, even after a surprise Ukrainian push into Russia’s Kursk region last month caught Russian forces off-guard.

Russia has also escalated its drone and missile attacks on Ukraine in recent weeks, while Ukraine has fired hundreds of long-range attack drones into Russian territory.

Later this month, Zelenskyy will travel to the US and will present a plan to Biden, and the two contenders in November’s presidential election, that he hopes will bring the end of the war closer.