Nato's wider actions - it also plans to open a training centre in Georgia and support for the reform of Ukraine's military - all ring alarm bells in Moscow.
Tensions could get worse still if the US or other Nato allies move to arm the Ukrainian military.
This is not a Nato issue as such but something for national governments, and everyone is watching the course of the evolving debate within the Obama administration.
Mr Stoltenberg said it will be the biggest reinforcement of its collective defence since the end of the Cold War.
Meanwhile French President Francois Hollande and German Chancellor Angela Merkel have visited the Ukrainian capital Kiev to present a new peace initiative. They are due to travel to Moscow on Friday.
US Secretary of State John Kerry is also in Kiev, and said President Obama was still "reviewing all options", including the possibility of providing "defensive weapons" to Ukraine.
The US has so far only provided "non-lethal" assistance to Ukraine.
Mr Stoltenberg said it was "up to different allies to decide" whether to arm Ukraine.
Fighting has intensified in recent weeks and the frequency of indiscriminate shelling in civilian areas has increased, according to the OSCE monitoring group.