VILNIUS - 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 Moscow provoked the worst East-West crisis since the Cold War by seizing Ukraine’s Crimea region. Taken together, the two announcements could point to a slight easing of tensions between the Western military alliance and Russia over Moscow’s annexation of Crimea and what NATO sees as Russian interference in eastern Ukraine.

NATO suspended all practical cooperation with Russia in April in protest at its annexation of Crimea, but left the door open to contacts at ambassadorial level or higher, to allow the two sides to discuss ways out of the crisis. 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. Rasmussen’s assessment was in line with earlier comments by a U.S. defence official that Russia had withdrawn most of its troops from the Ukrainian border, but that seven battalions, amounting to thousands of men, remained.

NATO estimated that, at the 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 A NATO military officer said on Wednesday that thousands of them had withdrawn, but tens of thousands remained.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 substantial forces along the Ukrainian borders that were ready to intervene if ordered to do so by Moscow. ‘We welcome what we have seen but we continue to urge Russia to pull back all troops from the Ukrainian border,’ Rasmussen told Reuters. In comments to a Lithuanian broadcaster, Rasmussen called on Moscow to ‘stop supporting armed pro-Russian gangs and seal the border so that we don’t see arms and fighters crossing into Ukraine.’

Ukraine’s government said on Friday it would press ahead with a military offensive against separatists, despite a deadly attack on an army helicopter amid reports that fighters from Russia have been involved in rebellions in the east. Answering questions from members of the NATO Parliamentary Assembly, made up of national members of parliament from the 28 NATO states, Rasmussen said NATO must adapt to the new kind of warfare Russia had practiced in Ukraine, which he said included covert military operations and disinformation campaigns.

All or part of NATO’s rapid reaction force, the NATO Response Force, should be on very high alert and ready to deploy very quickly, and the role of allied special operations forces would be even more important in future, Rasmussen said.