Rose Gottemoeller, U.S. assistant secretary of state for verification and compliance, said the meeting in Rome was "very productive" and "got off to a fast start."
Her Russian counterpart, Anatoly Antonov, who unexpectedly held a joint news conference with Gottemoeller at the U.S. Embassy in Rome, said Moscow would do its "utmost" to prepare a new draft treaty by year-end.
"We are sure, we are sure that this treaty, new treaty, will help to improve relations between [the] United States and [the] Russian Federation," Antonov said, speaking in English.
"We are sure that this treaty could promote confidence, predictability in the world. And I hope that it will be a very impressive impulse to international movement regarding getting rid [of the world's] nuclear weapon[s]."
The Rome talks were called after Presidents Barack Obama and Dmitry Medvedev agreed in London earlier this month to work out a replacement for the Strategic Arms Reduction Treaty, which expires in December.
Both sides see a new arms reduction deal as a way to show that the former Cold War foes can work together despite bitter disputes on other issues, such as NATO expansion into regions once dominated by Moscow and tackling Iran's nuclear ambitions.
"These negotiations will be very important in hitting the reset button in the U.S.-Russian relationship, restoring mutual confidence to make progress in a lot of areas," Gottemoeller said.
After Friday's talks, the first full-fledged negotiations will be held in May in the United States, Antonov said.
Gottemoeller, former director of the Carnegie Moscow Center, knew Antonov very well before the Rome talks, officials say, and the two demonstrated a certain rapport.
Calling each other by their first names, they fielded questions in both Russian and English and were positive about future talks even as they warned that the real negotiating work still lay ahead.
Sticking points include U.S. plans for a missile shield, which Russia opposes and which Antonov told reporters "is a very important issue" to Moscow.
Obama and Medvedev have ordered the negotiators to report back on progress by July, when Obama will make his first visit to Russia. Gottemoeller expressed optimism.
"On the basis of this very productive meeting today, I would say that we will have a good report for them in July," she said.
She told a conference earlier this month that it would be hard to meet a deadline of Dec. 5, when the existing START-1 treaty expires.
"We are very much satisfied to see Rose as the chief negotiator," Antonov said.
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