MOSCOW - President Dmitry A. Medvedev on Thursday welcomed the U.S. Senate's decision to ratify a landmark U.S.-Russian arms-control treaty, but Russian legislators said they needed to study a resolution accompanying the document before following suit.

Medvedev's spokeswoman, Natalya Timakova, said that when he signed the New Start treaty with President Obama, they agreed that the ratification process should be conducted simultaneously.

She said Medvedev voiced hope that both houses of the Russian parliament would ratify the pact but added that they would need some time to analyze the Senate's conditions for its ratification.

In a phone conversation Thursday morning, Medvedev congratulated Obama on the Senate's approval of the treaty, and the two leaders agreed that this was a historic event for both countries and for U.S.-Russia relations, according to a statement from the White House.

The New Start treaty, signed by Obama and Medvedev in April, would limit each country's strategic nuclear warheads to 1,550, down from the current ceiling of 2,200. It also would reestablish a system for monitoring and verification that ended last year with the expiration of a previous arms-control deal.

Legislators in the Kremlin-controlled parliament had said before the Senate landmark ruling Wednesday that they would approve the treaty quickly after it was ratified in the United States.

Lower house speaker Boris Gryzlov, however, told reporters Thursday that the Senate's ratification resolution contained some conditions and that the legislators needed to carefully study the text before making a decision.

He added that the State Duma may ratify the pact Friday if the text of the treaty itself remained unchanged.

He said that the house would need more time if it found any changes in the body of the treaty.

Russian lawmakers might need to work on the treaty until January, said Konstantin Kosachyov, the chairman of the international affairs committee in the State Duma. The resolution on the treaty's ratification "contains many interpretations that need a thorough study and a response of Russian lawmakers," he said.

Conservative Republicans said the pact would limit U.S. options on missile defense, lacked sufficient procedures to verify Russia's adherence, and deserved more time for consideration.

Obama called the treaty a national-security imperative and pressed strongly for its approval before Congress, with a Republican majority, assumes power in January. In recent days, he had telephoned a handful of wavering Republicans, eventually locking in their votes.

The Obama administration has argued that the United States must show credibility in its improved relations with its former Cold War foe, and the treaty was critical to any rapprochement. The White House is also counting on Russia to help pressure Iran over its nuclear ambitions.