NATO allies should "consider" lifting restrictions on the use of NATO weapons by Ukraine to hit targets in Russian territory, the alliance's secretary general said on Thursday.

"I believe that the time has come to consider some of these restrictions to enable the Ukrainians to really defend themselves," Jens Stoltenberg said in a speech in Prague, ahead of a meeting of NATO foreign ministers.

Some NATO countries supply arms to Ukraine without restrictions, but others stipulate they must only be used against targets on Ukrainian territory.

Stoltenberg said that as the war has moved closer to the Russian frontier, Russia has been able to attack Ukrainian forces from both sides of the border.

He also highlighted shortcomings in NATO support so far. "Over the last months, we have seen serious delays in the provision of ammunition military support, and serious gaps, for instance, when it comes to air defence and ammunition," Stoltenberg said.