Col Gen Oleksandr Syrskyi said Russia was continuing to send additional regiments and brigades from other areas and from training grounds to bulk up its troops on two main lines of attack in Kharkiv region’s north.

"The Polish position is that there should be no restrictions in this regard," said Cezary Tomczyk, deputy defense minster.

NATO Secretary General Jens Stoltenberg this week called on alliance members supplying weapons to Ukraine to lift restrictions on Kyiv using them to strike military targets inside Russia.