President Vladimir Putin proposed in late June that talks be opened with Washington on replacing the START-I treaty, which was signed at the end of the Soviet era and which expires in 2009, with a new arms deal.
"I doubt that either the Americans or we are ready for this or need it," the source said.
He said the countries would be unable to work out a new deal similar to START-I with mechanisms of strict control and verification of strategic potentials and nuclear warheads. "I don't see the point of doing so now that nuclear warheads are not and will not be counted," he said.
START-I was followed by START-II, which banned the use of multiple re-entry vehicles but never entered into force.
The source said that although old agreements were ineffective, general rules had been fixed in the Strategic Offensive Reductions Treaty, signed on May 24, 2002 by Vladimir Putin and George W. Bush in Moscow.
SORT, which expires on December 31, 2012, limited both countries' nuclear arsenals to 1,700-2,200 warheads each. The treaty has been criticized for a lack of verification provisions and the possibility of re-deploying stored warheads.
According to latest inspection reports, Russia possesses 927 nuclear delivery vehicles and 4,279 nuclear warheads for strategic offensive weapons, whereas the United States had 1,255 and 5,966 respectively.
The source said 1,500-2,000 warheads would be enough for Russia and that even 400-500 warheads were a substantial force against an enemy with an arsenal five times as large.
The spokesperson added that neither of the parties was eager to dispose of enriched plutonium or uranium, which are nuclear components of the weapons, because huge funds had been spent on enrichment programs.