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MOSCOW (AP) _ U.S.-Russian talks on nuclear arms cuts remain difficult, the top Russian negotiator said Monday amid intensified efforts to clinch an agreement before President Bush visits Russia next week.

Undersecretary of State John Bolton and Russian Deputy Foreign Minister Georgy Mamedov met in Moscow for the latest round of consultations on weapons reductions outlined last year.

``It's difficult,'' Mamedov said, according to the Interfax news agency.

The arms control agreement would require each country to cut its nuclear arsenal to 1,700 to 2,200 warheads from the 6,000 now allowed by the START I treaty.

Bush and Russian President Vladimir Putin agreed to those levels last fall and negotiators have been trying to work out a formal document codifying them in time for the May 23-26 summit.

Despite lingering differences, Secretary of State Colin Powell expressed confidence Sunday that Bush and Putin would sign an agreement on nuclear arms cuts during Bush's visit.

Mamedov said the agreement includes a clause allowing either party to pull out ``in case of a threat to national interests.'' He said the delegations were discussing specifics of the deal Monday along with overall strategic stability issues.

Mamedov did not say what was making the talks difficult. Russia's Interfax and ITAR-Tass news agencies cited unnamed diplomatic sources as saying the main remaining disagreement is over what to do with delivery vehicles.

Under START I, counting was based on warhead delivery systems, not the number of warheads. Russia wants to continue that method, but the current document specifies only the number of warheads to be cut, ITAR-Tass said.

Russia also wants the decommissioned warheads to be destroyed, while the United States wants to be allowed to store them in case of radical changes in the security environment in the future.

Powell and Russian Foreign Minister Igor Ivanov are to meet Tuesday and Wednesday in Reykjavik, Iceland, where they will attend a meeting intended to elaborate a new relationship between NATO and Russia.