"The time has come for allies to consider whether they should lift some of the restrictions they have put on the use of weapons they have donated to Ukraine," Stoltenberg said in an interview with the Economist. "Especially now when a lot of the fighting is going on in Kharkov, close to the border [with Russia], to deny Ukraine the possibility of using these weapons against legitimate military targets on Russian territory makes it very hard for them to defend themselves."

However, Stoltenberg dismissed the idea that NATO members can use their air defense systems to shoot down Russian missiles over Ukraine. He also spoke against the alliance's boots on the ground in Ukraine. "We don't have any intention to send NATO ground troops into Ukraine," he said. "We will not be party to the conflict."

Earlier, Russian Foreign Minister Sergey Lavrov said that Western weapons are already being used on Russian soil.