Both countries want to reduce their arsenals of strategic nuclear weapons
US and Russian negotiators have begun to work on a new treaty aimed at reducing the number of nuclear weapons.
The talks in Rome are the first step towards replacing the Strategic Arms Reduction Treaty (Start I), signed in 1991, which runs out in December.
Presidents Barack Obama and Dmitry Medvedev agreed to the talks at their first meeting earlier this month.
But there are areas of disagreement and meeting the December deadline will be difficult, correspondents say.
In particular, Moscow has expressed concern at US plans to build an anti-missile system in Central Europe.
"One should bear in mind that the lower we go in terms of the numbers of warheads, the more serious issues linked to missile defence and the strategic potential of other nuclear powers appear," Deputy Foreign Minister Sergei Ryabkov told the Russian news agency Interfax.
Russia has also said it would like to see a cut in delivery systems, such as rockets and submarines, not just warheads - an area not covered by existing agreements.
At a joint news conference at the US embassy in Rome, the heads of the US and Russian delegations said the first round of talks had been "productive".
Rose Gottemoeller, US Assistant Secretary of State for Verification and Compliance, said the meeting had "got off to a fast start".
"These negotiations will be very important in hitting the reset button in the US-Russian relationship, restoring mutual confidence to make progress in a lot of areas," she added.
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
Her Russian counterpart Anatoly Antonov, director of the Russian foreign ministry's Department of Security and Disarmament, said Moscow would do its "utmost" to prepare a new draft treaty before the end of the year.
"We are sure that this new treaty will help to improve relations between the United States and the Russian Federation," he said.
"We are sure that this treaty could promote confidence and predictability in the world."
Mr Antonov said the first full negotiations were scheduled to begin in the US in May, when Russian Foreign Minister Sergei Lavrov and US Secretary of State Hillary Clinton will meet.
But both sides want to reduce their nuclear arsenals and agree a replacement for Start I, the BBC's Duncan Kennedy in Rome says.
The US in particular believes it will give them greater moral and political force against countries with nuclear ambitions such as Iran and North Korea, our correspondent adds.
Both presidents want the new deal to improve on an agreement by their predecessors in 2002 to cut deployed warheads to between 1,700 and 2,200 on each side by 2012.
They have asked their negotiators to report on their progress by July.