"The time has come to consider whether it will be right to lift some of the restrictions which have been imposed because we see now that especially in the Kharkiv region the front line and the border line is more or less the same," Stoltenberg told NATO lawmakers meeting in Bulgaria.

"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."

Ukraine has been pressing its Western backers -- especially the United States -- to allow it to use the longer-range weaponry they supply to hit targets in Russia.

Washington, and other allies, have been reluctant to permit Kyiv to strike over the border out of fear that it could drag them closer to direct conflict with Moscow.

Stoltenberg said "some allies have lifted restrictions enabling the Ukrainians to better defend themselves", without giving details.

"It is clear that Ukraine has the right to defend themselves," the secretry general of the Western alliance said.

"Self-defence includes the right to also attack legitimate military targets inside Russia."