The summit in Wales will specifically bolster NATO's rapid response force, creating a spearhead of "several thousand troops" which could be deployed within "very few days" to meet any new threat, Rasmussen told a news conference in Brussels.
This will "ensure that we have the right forces and the right equipment in the right place, at the right time," he said.
"That also means a more visible NATO presence in the East for as long as required."
In response, NATO has rotated small numbers of troops and aircraft through the region to reassure its allies and Washington has been at pains to stress that the alliance will honour its commitment to help any ally coming under attack.
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Rasmussen made the point again today.
The new measures were being taken "not because NATO wants to attack anyone but because the dangers and the threats are more present and more visible ... We will do what it takes to defend our allies."
Since the troops would not be permanently based there, it would not breach the terms of the 1997 NATO-Russia Founding Act, which fixed Europe's post-Cold War borders, he said.
Rasmussen repeated that NATO remained committed to the Founding Act provisions, which laid down the need for peaceful change in international borders, while Russia was "in blatant breach."
The NATO summit Thursday and Friday is expected to be dominated by the Ukraine crisis, which Rasmussen and many others see as the most severe threat to Euro-Atlantic security in a generation.