Russia and the United States will pursue a new deal to cut nuclear warheads, presidents Dmitry Medvedev and Barack Obama said on Wednesday, making good on a pledge to rebuild relations from a post-Cold War low.

They said in a joint statement they had ordered negotiators to report first results by July, when Obama will visit Moscow for a summit.

In the past years, there were strains in relations between our two countries and they were drifting in the wrong direction, Medvedev told reporters.

This was not in the interests of the United States, Russia or global stability. We agreed to open a new page in these relations, to reset them, given the joint responsibilities of our states for the situation in the world.

Obama said he and Medvedev had begun constructive dialogue on issues from nuclear proliferation to counter-terrorism to economic stability.

The two leaders, both in their 40s, were meeting for the first time in London on the eve of a G20 summit to discuss the global economic crisis.

They acknowledged lingering differences over last year's Russia-Georgia war and over U.S. proposals to base parts of a missile shield in Eastern Europe, something Moscow considers a threat to its security.

Those disagreements have badly damaged relations, but the two countries have vowed to press the reset button on ties -- an aim the planned arms treaty could help them to achieve.

The new agreement will mutually enhance the security of the parties and predictability and stability in strategic offensive forces, they said in a joint statement.

We are ready to move beyond Cold War mentalities and chart a fresh start in relations between our two countries.

The proposed arms deal would go beyond the 2002 Strategic Offensive Reductions Treaty (SORT), which committed both sides to cutting arsenals to between 1,700 and 2,200 warheads by 2012.

It would replace the 1991 Strategic Arms Reduction Treaty (START 1), which led to the biggest ever bilateral cuts in nuclear weapons, but will expire in December.

Russia sees the treaty as the cornerstone of post-Cold War arms control and believes that letting it lapse with no replacement could upset the strategic balance.

Among difficult issues to be negotiated will be differences over the way warheads are counted, and whether warheads removed from missiles should be stored or destroyed.

Moscow wants to link the successor treaty to the proposed U.S. missile shield plan, which it strongly opposes.

The two leaders agreed to work together on Afghanistan, urged Iran to restore confidence in the peaceful nature of its atomic program, and expressed concern about an upcoming North Korean rocket launch.