The change in American policy is understood to apply only to defend the area of Kharkiv, according to reports.

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It comes as Ukrainian officials stepped up calls on the US administration to allow its forces to defend itself against Russian attacks.

Ukrainian President Volodymyr Zelenskyy and others have been increasingly vocal in making the case that the restriction was putting Ukrainian forces in an untenable situation as Russia has intensified attacks around the northeast Kharkiv region.

Russia made advances in the area during a lengthy delay in replenishment of US military aid and as western Europe's inadequate military production slowed crucial deliveries to the battlefield for Ukraine.

Kharkiv, Ukraine's second-largest city, is just 20 kilometres (12 miles) from the Russian border.

But US officials underscored that the US policy calling on Ukraine not to use American long-range missiles and other munitions to strike inside Russia offensively has not changed.

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Antony Blinken, the US secretary of state, had earlier hinted about the change in policy at a meeting of NATO foreign ministers in Prague on Thursday.

There, he said Moscow's use of misinformation and disinformation was a "poison" and signed an agreement with the Czech government to combat it.

Since the start of the Russian invasion in February 2022, Biden has been steadfast in his opposition to Ukraine's offensive use of American-made weaponry, concerned that the action could be seen as provocative and lead to Moscow widening the war.

But calls for a change in policy have been mounting.

Earlier this week, French President Emmanuel Macron and NATO chief Jens Stoltenberg said that Western countries should not object if Ukraine needs to strike inside Russia to defend itself.