Russia was on course Friday to ratify a landmark nuclear disarmament pact by the end of the month after introducing non-binding amendments that countered those made by the US Senate.

A top lawmaker said one of those additions spelled out Moscow's right to withdraw from the treaty if Washington ever deployed a missile defence shield powerful enough to resist Russia's vast nuclear arsenal.

Russia's State Duma lower house of parliament easily passed the new START agreement -- the first between the two former Cold War foes in two decades -- in an initial vote held December 24.

But lawmakers have since added some 20 pages to the ratification document that are meant as a counterpoint to the changes that US senators made before ratifying the treaty after a heated debate last month.

A top Duma deputy said those revisions now suited the pro-Kremlin party that dominates the chamber and that lawmakers would have no problem approving the pact before passing it on to the upper house -- where quick passage is certain.

"I think that our third reading will happen on January 25," Konstantin Kosachev told Moscow Echo radio.

"In all respects ... this agreement is necessary, important, useful and advantageous to Russia," he added.

The ruling United Russia party of which Kosachev is a member has the necessary votes to the pass the pact without support from the opposition Communist Party that initially voted against the pact.

The agreement limits each side to 1,550 deployed warheads and 700 deployed long-range missiles -- including those fired from submarines -- and heavy bombers.

The two nuclear superpowers may also have up to 800 deployed and non-deployed launchers and bombers.

But US senators argue that the treaty does nothing to prevent Washington from deploying a new missile defence system that Moscow fears may one day be expanded to hurt its own nuclear capabilities.

The senators thus introduced language confirming the United States' right to deploy such a system and also arm long-range missiles with conventional warheads that Russia fears may be used in regional wars.

Russia insists that those weapons should also be counted under the treaty's terms because they are impossible to distinguish from nuclear-tipped missiles while in flight.

"Should we take the Americans at their word when they tell us that those carriers are armed with conventional warheads?" Kosachev asked. "Of course not."

Kosachev said the changes were meant primarily as a warning and should not be treated as Duma resistance to a pact that Russian President Dmitry Medvedev and US President Barack Obama signed under much fanfare in Prague on April 8.

He conceded that the defence shield idea "does not threaten our capabilities in the matter in which it is being announced (by the United States) today."

"But what follows ... are exceptional circumstances under which the treaty may cease to exist," Kosachev added.

The White House argues that the shield is meant to protect its allies in Europe from missiles that might one day come in from North Korea or Iran.

But Kosachev said nothing can at this stage prevent the United States from making the shield far more efficient.

"So even now, at this stage, we have honestly warned the Americans that one of the exceptional circumstances includes qualitative change in the nature of the US anti-missile defences."

He added that some US senators were calling on Obama "not to give in to the Russians and to develop these systems in full."