Russia also wants warhead delivery systems included in the new deal
Russia and the US have been holding the first of three days of talks in Moscow on a new treaty aimed at reducing their stockpiles of nuclear weapons.
Senior diplomats need to negotiate a replacement for the Strategic Arms Reduction Treaty (Start I) of 1991, which expires on 5 December.
The main sticking points are limits on the number of warheads and whether the treaty will cover bombers and missiles.
Also on the agenda is Moscow's concern over US missile defence shield plans.
Analysts say a successful outcome would be a boost for relations before a visit to Moscow in July by US President Barack Obama.
Mr Obama and his Russian counterpart, Dmitry Medvedev, agreed last month to pursue a deal to replace Start I.
The two-day negotiations began on Tuesday behind closed doors at a 19th Century foreign ministry mansion in Moscow and will continue on Wednesday as planned.
The US team is being led by Assistant Secretary of State Rose Gottemoeller and include officials from the Pentagon and the US Department of Energy.
Ms Gottemoeller, a Russian expert, held preliminary talks in Rome last month with Russia's chief negotiator Anatoly Antonov.
A Russian foreign ministry spokesman said that both sides had agreed the talks would be discreet and that they would only release a joint statement at the end.
Ahead of the meeting, Russian Foreign Minister Sergei Lavrov said he hoped the negotiations would be "fruitful", but warned that they would be linked to US plans to build an anti-missile system in central Europe.
US-RUSSIA ARMS ACCORDS
1972: Salt I treaty agrees to freeze levels of strategic nuclear missile launchers and submarines; Anti-Ballistic Missile treaty bans missile-defence systems
1979: Salt II Treaty imposes for first time ceiling on strategic nuclear weapons
1987: INF treaty eliminates short and medium-range missiles
1991: Start I slashes nuclear arsenals by one-third
1993: Start II treaty reduces number of nuclear warheads and bans use of MIRV warheads
1999: US Senate blocks nuclear test ban treaty
2000: Russian parliament ratifies Start II treaty
2002: US withdraws from ABM Treaty; Russia withdraws from Start II the following day
2002: Sort treaty commits both sides to cut arsenals by two-thirds
"We believe that the Start treaty cannot be discussed in a vacuum," he was quoted as telling Russian media on Monday.
"It must reflect the issue of global security, which certainly includes Russia's, and this implies that we must sort out the situation on missile defence."
The US says the missile defence system is intended to destroy incoming ballistic missiles fired by "rogue states", such as North Korea and Iran.
Moscow has also said it would like to see a cut in so-called delivery systems, such as missiles, bombers and submarines, not just nuclear warheads - an area not covered by existing agreements.
Meanwhile, the US is reportedly prepared to count only the warheads ready for launch, while Russia wants to count those in storage as well.
The US also plans to swap nuclear warheads for conventional ones on some ballistic missiles. Russia says they would be impossible to differentiate.
But both want the new deal to improve on the 2002 Treaty of Moscow, which will cut deployed warheads to between 1,700 and 2,200 on each side by 2012.