NATO Secretary General Jens Stoltenberg described what he thinks a de-escalation of Russian forces near Ukraine's borders should look like.

"What we should see is, of course, a significant withdrawal of troops, but not only troops, but also equipment. Because last spring we saw them moving in with troops and heavy equipment," Stoltenberg said.

It very often takes much longer to move heavy military equipment than it does to move troops in and out, he said.

"And what we've seen before is that they come in with heavy equipment and troops, take some troops out, and then they can easily bring them back in just a few days, in a very short period of time," the secretary general said.

"So we need to see a significant and prolonged withdrawal of forces, troops and not least heavy equipment," he added.