Putin made two points about this in June, 2016[0]. First, it's not possible to verify what's in the warheads of anti-ballistic missiles. Second, defense and offense are part of a single set of capabilities. If you remove the opponent's ability to retaliate against a first strike, that is just as dangerous to them as if you had increased your offensive capability. I'm well out of my depth in terms of judging if these statements are true, but this is what he said.

[0] https://www.youtube.com/watch?v=kqD8lIdIMRo