Russia and the US are to reopen negotiations about reducing their nuclear warheads, presidents Dmitry Medvedev and Barack Obama have said.
The discussions will be the first such talks for more than a decade.
Mr Obama said earlier there were very real differences between Washington and Moscow, but that there was also a broad set of common interests.
The announcement came on the fringes of the G20 summit of world leaders which is convening in London.
Mr Medvedev has invited his American counterpart to visit Moscow in July - an invitation Mr Obama has accepted.
Russia and the US have also agreed to discuss "mutual international co-operation", the two presidents said.
After their meeting, Mr Medvedev said he viewed prospects for future bilateral relations "with optimism".
President Obama said they had begun a
In a joint statement, the two leaders said they had ordered nuclear negotiators to report first results in July.
Mr Obama said the US and Russia would be in a much stronger position to strengthen the non-proliferation treaty if they were to lead by example and reduce their own nuclear arsenals.
It is hoped the talks will produce a new arms control treaty to replace the Strategic Arms Reduction Treaty (Start) that expires at the end of the year.
Agreed in 1991, Start limits the world's two largest nuclear arsenals to between 1,700 and 2,200 warheads.
This is the first tangible sign of the "re-setting" of the relationship between Moscow and Washington heralded by the Obama administration taking office, says the BBC's diplomatic correspondent, Jonathan Marcus.
This development is important, our correspondent adds, because the hope is that if the nuclear weapons states give up more of their warheads, it will assist efforts to bolster the increasingly threadbare regime that seeks to guard against the spread of nuclear arms.
The presidents' statement also called on Iran to co-operate with the UN nuclear watchdog and prove its nuclear programme is for peaceful purposes - not for making atomic weapons.
The two presidents also agreed to work together on Afghanistan and expressed concern about an upcoming North Korean rocket launch.
World leaders are gathering in London to discuss ways to resolve the financial crisis, which Mr Obama has said was the most severe since World War II.
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