The lower house of the Russian parliament, the Duma, has voted to give initial approval to the Start nuclear arms pact with the US.
The treaty, aimed at reducing both countries' nuclear arsenals and allowing their inspection, was ratified by the US Senate on Wednesday.
There were hopes the Duma would give its full approval but officials decided that two more readings were necessary.
The pact must also be ratified by the upper house, the Federation Council.
Before MPs voted to back the treaty by 350 votes to 58, concerns were expressed in the Duma about two non-binding amendments that had been made by the Senate before ratification.
Russian Foreign Minister Sergei Lavrov said the amendments - committing the US to a missile defence system and modernising its nuclear arsenal - did not change the treaty as a whole.
He said US efforts to remove "future strategic-range non-nuclear systems" from the scope of the pact were "unacceptable".