Vilnius, Reuters—Russia is in the process of pulling back around two thirds of the troops it had close to the border with Ukraine, NATO Secretary-General Anders Fogh Rasmussen said on Friday, a far more significant withdrawal than NATO has previously estimated.

Rasmussen also announced that ambassadors from Russia and NATO countries would meet in Brussels on Monday for the first time since March 5, soon after Russia provoked the worst East-West crisis since the Cold War by seizing Ukraine’s Crimea region.

Monday’s meeting is expected to discuss the security situation in and around Ukraine, a NATO official said.

“We have seen signs of at least a partial withdrawal. Our estimate is that around two-thirds of Russian troops have been or are being pulled back,” Rasmussen told Reuters on the sidelines of a meeting of the NATO Parliamentary Assembly in the Lithuanian capital, Vilnius.

NATO has estimated that, at their peak, Russia had around 40,000 troops massed close to the Ukraine border. A NATO military officer said on Wednesday that thousands of them had withdrawn, but tens of thousands remained.

Despite the withdrawal, Rasmussen said that Russia still had a substantial number of forces along the Ukrainian borders that were ready to intervene if ordered by Moscow.

“We welcome what we have seen but we continue to urge Russia to pull back all troops from the Ukrainian border,” Rasmussen said.