Biden had come under increasing pressure from a desperate Ukraine to ease his ban, but had so far resisted amid fears it could drag NATO into direct conflict or even a nuclear standoff with Moscow.

"The president recently directed his team to ensure that Ukraine is able to use US-supplied weapons for counter-fire purposes in the Kharkiv region so Ukraine can hit back against Russian forces that are attacking them or preparing to attack them," a US official said.

"Our policy with respect to prohibiting the use of ATACMS or long range strikes inside of Russia has not changed," the official said on condition of anonymity, referring to long-range missiles recently sent by Washington to Kyiv.

A second US official confirmed Biden's change of policy, which comes as Russian forces push forward with an offensive against the northeastern Kharkiv region.

Ukrainian President Volodymyr Zelensky has been pressing Kyiv's supporters -- chiefly the United States -- to allow it to use the longer-range weaponry they supply to hit targets on Russian soil.

The United States has been rushing to get arms to Ukraine after Congress passed a huge military aid package following a six-month delay in April, but Biden had been reluctant to allow cross-border strikes.

However US Secretary of State Antony Blinken had hinted on Wednesday that Biden was changing course in response, saying that Washington had "adapted and adjusted."

Biden's shift followed weeks of behind-the-scenes discussions between the White House and top US military and State Department officials in the days after Russia launched its Kharkiv offensive on May 10.

Ukraine asked for permission to use US arms on Russian territory on May 13, and Biden agreed in principle on May 15 after National Security Advisor Jake Sullivan made the case for Ukraine to be able to defend itself.

- 'Neutralize' -

Biden then met on May 17 with Blinken, who traveled to Kyiv earlier this month to see the increasingly grave situation as Russia pushes towards Kharkiv, and had been widely reported to be pressing Biden to ease the rules.

After working through the details, the US president gave a final sign-off in recent days but the decision was kept secret for operational reasons and only became effective on Thursday.

The White House has however made it clear to Kyiv that the permission for cross-border attacks was limited to targets just over the border, and which were being used by Russia to launch offensives.

Officials also insisted that they had been expecting some form of Russian offensive, and that with the influx of US arms, it has been stalled, preventing the possible capture of Kharkiv city.

Pressure had been mounting for Biden and other Western countries to end their ban on Ukraine using their weapons to strike Russian soil.

Ahead of a meeting of NATO foreign ministers in Prague, alliance chief Jens Stoltenberg said repeatedly it was time for members to reconsider those limits.

French President Emmanuel Macron also appeared to shift the dial forward on Tuesday when he said Ukraine should be allowed to "neutralize" bases in Russia used to launch strikes.

Some countries including Britain and the Netherlands had already said Kyiv has the right to use their weapons to strike military targets in Russia.

Biden's change also comes shortly before he heads to Europe for a series of meetings where Kyiv's plight will be in focus.

Biden will attend ceremonies in France marking the World War II D-Day landings in early June where Ukraine's Zelensky will also be present.

The US president will also meet leaders of the world's top economies at the G7 summit in Italy.

But Zelensky has criticized Biden for apparently planning to skip a Ukraine peace summit in Switzerland just after the G7, which clashes with an election fundraiser for the US president hosted by Hollywood stars George Clooney and Julia Roberts.