Joe Biden has allowed Ukraine to use some US-made weapons over one part of the Russian border, to allow Kyiv’s forces to defend against an offensive aimed at the city of Kharkiv, relaxing an important constraint on Ukraine’s able to defend itself.

“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.

Limits on the use of US long-range weapons such as the army tactical missile system (Atacms) will remain, however.

“Our policy with respect to prohibiting the use of Atacms or long-range strikes inside of Russia has not changed,” the official said.

The statement suggests that the main impact of the policy change would be to allow Ukrainian US-supplied artillery to fire back at Russian batteries over the Russian border from Kharkiv region, which have had a withering effect on Ukrainian defences around Kharkiv city.

The change will also allow the Ukrainian army to target concentrations of Russian forces massing on the border in Russia’s Belgorod region.

Biden’s decision followed calls from US allies in Europe, including the UK, Germany and France, and the Nato secretary general, for Ukraine to be able to use western-provided weapons against military targets in Russia. The White House was also under pressure from Ukraine supporters in both parties in Congress.

Vladimir Putin warned of “serious consequences” if Russia is struck with western weapons.

The change of policy began just after Russia launched its new Kharkiv offensive. According to sources familiar with the decision, the Ukrainians made an appeal in a secure video conference on 13 May with the US national security adviser, Jake Sullivan, the defence secretary, Lloyd Austin and the chairman of the joint chiefs of staff, Gen Charles Brown. Austin, Brown and Sullivan then agreed to recommend the change, and Sullivan took the proposal to Biden on 15 May.

Biden agreed as long as the Ukrainian use of US weapons across the border was focused on and limited to the forces directly attacking them. After the secretary of state, Antony Blinken, returned from Kyiv, he was consulted and agreed. Since then, the administration has been focused on honing the new policy with the aim of making sure unforeseen consequences are contained as much as possible.

“I think the immediate change is that Ukraine will be able to use Himars [high mobility artillery rocket systems], which means they can strike targets more effectively at greater depths,” said Rob Lee, a senior fellow in the Foreign Policy Research Institute’s Eurasia program.

Ukraine has been suffering higher numbers of losses to its artillery along the frontlines in Kharkiv in recent weeks, said Lee, probably because they had to operate closer to the frontlines and Russia had been successfully targeting them with Lancet loitering munitions.

Ukraine could use the decision to use Himars to target Russian artillery, multiple rocket systems, UAV teams, Lancet loitering munitions teams, electronic warfare systems, and command and control posts, said Lee.

He said: “It’s not clear we’re going to see a change on the frontlines in Kharkiv. But it will make it more difficult for Russia to continue offensive operations in that direction. So it will make it easier for Ukraine to defend Kharkiv.”

Early on Friday, Russian forces shelled a five-storey apartment building in Kharkiv, starting a fire and wounding eight people, according to local officials who warned that residents could be trapped under rubble.

Several strikes were reported to have hit the area after the initial attack.

“A fire has broken out. People could be under the rubble,” the regional governor, Oleh Syniehubov, said of the attack, which occurred at about midnight local time.

The Biden administration believe that the resumption of US armed supplies in late April, after congressional Republicans dropped their opposition, combined with US advice on defensive measures, have helped to slow Russian progress. Administration officials think the new policy on the use of US weapons will help stop the offensive altogether.