President Barack Obama vowed Saturday to pursue the deployment of US missile defense systems and rejected Russia's claim that doing so would justify withdrawing from a new nuclear arms control treaty.

"Regardless of Russia's actions in this regard, as long as I am president, and as long as the Congress provides the necessary funding, the United States will continue to develop and deploy effective missile defenses to protect the United States, our deployed forces, and our allies and partners," he said.

Obama's strong message on an issue that has at times deeply angered Moscow came in a letter to top senators as his Republican foes called for killing the new Strategic Arms Reductions Treaty (START) over missile defense concerns.

Republicans have rallied behind an amendment by Republican Senator John McCain to strip out language in START's preamble tying offensive nuclear weapons to defensive systems.

The preamble is non-binding but, because it resulted from talks between Washington and Moscow, passing the amendment would have forced the accord back to the negotiating table, effectively killing the agreement.

The amendment would need nine of Obama's Democratic allies in the Senate to join all 42 Republicans to pass -- meaning the measure was all-but-certain to go down to defeat.

Republicans have pointed to Russia's unilateral statement, when the treaty was signed by Obama and Russian President Dmitry Medvedev in April, that a rise in US missile defenses could lead Moscow to back out of the accord.

Russia said reasons to quit the treaty under a clause saying that either side may do so if their national security is threatened "include a build-up" of US missile defenses "such that it would give rise to a threat to the strategic nuclear force potential of the Russian Federation."

"The United States did not and does not agree with the Russian statement," Obama said in the message, which the White House made public.

Republicans appeared to be caught off guard by the letter, which Democratic Senator John Kerry read to his colleagues, though McCain responded: "Presidents don't last forever, but binding treaties do."