US President Joe Biden administration's policy shift to allow the US weapons to be used by Ukraine against limited targets inside Russia is a dangerous escalation of the conflict, those opposed to the policy say.

Biden's directive allows for US-supplied weapons to be used for "counterfire purposes in the Kharkiv region so Ukraine can hit back against Russian forces that are attacking them or preparing to attack them", said one of four US officials who spoke to The Associated Press.

The officials, who requested anonymity, said the US policy calling on Ukraine not to use the US' ATACMS or long-range missiles and other munitions to strike inside Russia has not changed.

The news of the US policy shift was first reported on May 30 by Politico, which said that it was a result of recent Russian military gains around Kharkiv. Politico said the policy change was made "secretly".

Russian Deputy Foreign Minister Sergey Ryabkov warned on Monday the US could face "fatal consequences" if it ignored Moscow's warnings not to let Ukraine use weapons provided by Washington to strike targets inside Russia.

"I would like to warn American leaders against miscalculations that could have fatal consequences. For unknown reasons, they underestimate the seriousness of the rebuff they may receive," state news agency RIA quoted Ryabkov as saying.

He referred to comments last week by President Vladimir Putin, who said NATO countries were playing with fire and risking a deeper global conflict, one of a series of warnings from Moscow about the risk of a serious escalation.

Meanwhile, several commenters on social media have speculated the move was made while attention was focused on former US president Donald Trump's hush money trial in New York City.

"Domestic news may threaten to overshadow this very serious development, so timing is perhaps not accidental," wrote Aaron Kheriaty, a physician and writer, to his 299,000 followers on X.

"While the Trump indictment overwhelms the news, sounds like a serious escalation that will force a potentially devastating counter-move by Putin. Curious Timing," wrote Edward Dowd, founder of Phinance Technologies, to his 288,000 followers on X.

"This is what they are distracting us from with the Trump verdict!" wrote Winnie Schola, an architectural designer, on X.

Michael Tracey, in an X post to his 322,000 followers, wrote on May 21: "The hubris in Congress toward nuclear escalation is jaw-dropping. Bipartisan coalition demands Biden authorize Ukraine to strike inside Russia with US intel/weapons."

On Monday, Ukrainian forces said they had struck a Russian S-300 missile system using Western-supplied weapons inside Russian territory, although the origin of the weapons was unclear.

Agencies contributed to this story.

hengweili@chinadailyusa.com