The United States must allay Russian concerns over its planned anti-missile system in Europe if the two sides are to achieve a breakthrough on cutting nuclear weapons, Foreign Minister Sergei Lavrov said Wednesday.

U.S. President Barack Obama and President Dmitry Medvedev last month agreed to pursue a deal on cutting nuclear weapons that would replace the 1991 Strategic Arms Reduction Treaty, or START, which expires in December.

The world's two biggest nuclear powers began formal talks Tuesday in Moscow to find a replacement for START, and diplomats hope that progress can be made before Obama and Medvedev meet in Moscow from July 6 to 8.

But the talks are complicated by Washington's anti-missile plan. It is considering stationing elements of a missile shield in Poland and the Czech Republic in order to intercept rockets fired from what it regards as rogue states, such as Iran. Russia sees this as upsetting the strategic balance and threatening its own security.

"The final product of the negotiations must, of course, be a step forward from the current system of limits and cuts," Lavrov told reporters at the 19th-century mansion in central Moscow where talks on the successor to START continued Wednesday.

"The fundamental principle of an agreement must be equal security for both sides and the preservation of strategic parity. This, of course, cannot be ensured without taking into account the situation with anti-missile defense," Lavrov said.

He said the talks should also take account of any plans for space-based missiles and the development of highly destructive non-nuclear weapons.

News agencies quoted an unidentified Russian diplomat as saying Wednesday that the talks so far have been "successful and constructive."

Obama said last month that the United States would go ahead with the anti-missile system if Washington thought that there was a continued threat from Iran.

Obama said Tuesday that it is "absolutely imperative" that the United States take the lead in reducing the spread of nuclear weapons, particularly at a time when North Korea and Iran are developing nuclear weapons capabilities, nuclear-armed Pakistan and India remain in a long-running conflict and al-Qaida is trying to get such weapons.

"It is absolutely imperative that America takes leadership working with not just our Russian counterparts but countries all around the world to reduce and ultimately eliminate the dangers that are posed by nuclear weapons," Obama said in Washington.

Meanwhile, Security Council deputy head Vladimir Nazarov said Wednesday that Russia was concerned that Pakistani nuclear weapons could fall into the hands of terrorists.

Nazarov said the situation in Pakistan is becoming increasingly dangerous and suggested that the government is losing control, Interfax reported. Nazarov met Wednesday with counterparts from Shanghai Cooperation Organization's member nations.

(Reuters, AP)