MOSCOW – 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, Russia's foreign minister said on Wednesday.

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

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

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

Finding a deal could herald a thaw in relations between the Cold War foes after bitter arguments under former U.S. President George W. Bush over missile defense, NATO expansion and last August's war between Russia and Georgia.

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

The negotiators face tight pressure to work through scores of complicated technical issues -- including differences over how to count nuclear weapons and ensure compliance -- before the December 5 deadline for replacing START.