Thousands of Russian soldiers are forming a new battle group north-west of Kharkiv, Volodymyr Zelensky has said as he pleaded for permission to strike them with Western missiles.

The Ukrainian leader said that satellite images showed Russian forces were now deployed on the border near the Ukrainian city of Sumy.

“We can see every point where Russian troops are concentrated. We know all areas from which Russian missiles are launched and combat aircraft take off,” he said.

Ukrainian officials are frustrated that they are not allowed to strike targets inside Russia with Western missiles, essentially giving Russian forces a safe area to prepare ground attacks and to fire missiles at Ukrainian cities.

They have warned since a Russian attack towards Kharkiv on May 10 that the Kremlin also plans to attack Sumy, which lies 100 miles from Ukraine’s second city.

The US-based Institute for the Study of War confirmed the Russian troop build-up towards Sumy but said that the Kremlin’s plan may be to “fix and draw” Ukrainian forces defending the area around Kharkiv.

Pressure is building on the US to follow Britain and allow Ukrainian forces to strike targets inside Russia, despite concerns that this will escalate the war.

Over the weekend, Poland and Sweden gave permission for Ukraine to hit targets inside Russia and Jens Stoltenberg, the secretary general of Nato, has also backed the policy change.

“If [Ukraine] cannot attack military targets on Russian territory then it ties one hand of the Ukrainians on their back and makes it very hard for them to conduct defence,” he said at a Nato meeting in Sofia on Monday.

But Joe Biden has not yet been persuaded and Germany and Italy have both ruled it out.

Ukraine has been using domestic drones to hit targets across Russia, mainly oil refineries, railways and military airfields, but on Monday sources confirmed that it had struck a Russian nuclear radar station near the border with Kazakhstan, 1,000 miles away.

This is the second Ukrainian strike against Russia’s nuclear missile infrastructure in four days to the worry of  Kyiv’s Western allies.