"The evidence comes one piece after another, including that based on the data from our intelligence [services]. A lot of photos have been published. But if people do not believe our intelligence, our words, they can at least read the media, both Russian and foreign," Stoltenberg said.

"Moscow's position today reminds me of the situation when the Russian government tried to deny that it had sent troops to Crimea. They sent soldiers there in unmarked [uniforms], and said that there were no Russian soldiers in Crimea. And only after they had annexed the Ukrainian territory of Crimea did they admit that there was the direct participation of the Russian army," Stoltenberg said.

According to Stoltenberg, NATO has a large amount of different types of intelligence information that proves the Russia's military presence in eastern Ukraine.