First of all, the treaty is non-binding on the United States until it is ratified by 2/3s of the Senate. So any thing in the treaty can, and probably will be changed.
Second, there is nothing in the treaty that prohibits the United States from responding to any attack with nuclear weapons.
Third, statement about whether the US will use or not use nuclear weapons is documented in the Nuclear Posture Review (NPR). The NPR, for the first time clearly states the United States' position on this matter. Previous versions of the NPR were ambiguous. A summary of our position can be accessed at
http://www.fas.org/blog/ssp/2010/04/npr2010.php
- The United States will continue to strengthen conventional capabilities and reduce the role of nuclear weapons in deterring non-nuclear attack, with the objective of making deterrence of nuclear attack on the United States or our allies and partners the sole purpose of U.S. nuclear weapons.
(Nukes will be used to deter nukes. Not a bad policy)
- The United States would only consider the use of nuclear weapons in extreme circumstances to defense the vital interests of the United States or its allies and partners.
(You use chemical or biological weapons against us, we can nuke you)
- The United States will not use of threaten to use nuclear weapons against non-nuclear weapons states that are party to the NPT and in compliance with their nuclear proliferation obligations.
(if you play nice and follow the NPT rules, we won't threaten you with nuclear weapons. North Korea, Israel, and India, you might want to pay attention here)
Nothing in either the treaty nor the NPR prohibiting the use of nuclear weapons in response to a chemical or biological attack.