With a flick of their pens the world became a more peaceful place

Obama and Medvedev cut nuclear weapons but crank up the pressure on Iran

Barack Obama yesterday honoured a pledge to make the world safer by shrinking the global stockpile of nuclear weapons and sealing a dramatic turnaround in America's strained relations with Russia.

In the gilded splendour of Prague Castle, the US President joined his Russian counterpart, Dmitry Medvedev, to sign a treaty slashing their countries' deployed nuclear arsenals by a third, and to state their shared determination to prevent the proliferation of nuclear weapons.

"Today is an important milestone for nuclear security and non-proliferation, and for US-Russia relations," Mr Obama said of a deal that replaces the expired 1991 strategic arms reduction treaty (Start) and which he claimed "set the stage" for further cuts in atomic weapons.

Mr Medvedev, who smiled and joked with Mr Obama during the ceremony, insisted there were "no losers" in a pact that "fully reflects the interests of the United States and Russia". "I hope this signing will open a new page in relations between our countries," he said.

Russia's fear of a missile defence system that the US intends to build in central Europe was the main sticking point during a year of talks on the treaty. The Pentagon says the system will neutralise the threat of rocket attacks from Iran, but Moscow believes it could undermine its nuclear deterrent.

The Kremlin warned again yesterday that Russia could withdraw from the pact if it felt threatened by the US system. Russia's foreign minister, Sergei Lavrov, said Moscow could pull out of the treaty if "a quantitative and qualitative build-up of the US strategic anti-missile potential begins to significantly affect the efficiency of Russia's strategic forces".

The pact, the most significant nuclear disarmament treaty in a generation, shrinks the nuclear arsenal by allowing both sides a maximum of 1,550 strategic warheads over seven years, about a third less than currently permitted. It allows no more than 800 land, air and sea-based launchers.

But it also marks a two-pronged diplomatic approach that cranks up the collective pressure on Iran over its controversial atomic programme. "Regrettably, Tehran is not reacting to a range of suggested constructive compromise agreements. We can't close our eyes to this. That is why I do not exclude that the Security Council will have to examine this question again," Mr Medvedev said.

Mr Obama went further, emphasising that the treaty demonstrated US and Russian commitment to slashing their own atomic arsenals as well as to taking action to prevent the spread of nuclear weapons and material around the world. "That's why the United States and Russia are part of a coalition of nations insisting that the Islamic Republic of Iran face consequences, because they have continually failed to meet their obligations," he said.

"We are working together at the United Nations Security Council to pass strong sanctions on Iran and we will not tolerate actions that flout the NPT [nuclear non-proliferation treaty]," Mr Obama added. "My expectation is that we are going to be able to secure strong, tough sanctions on Iran this spring."

Earlier this week, the Obama administration declared it would not attack a non-nuclear country unless that state was not adhering to the NPT – thereby leaving open the option of a nuclear strike against Iran and North Korea.

As part of his drive to isolate Iran, which insists it only seeks atomic power and not weapons, Mr Obama will host a 47-nation summit next week on nuclear non-proliferation, and is expected to urge China's President Hu Jintao to back a tougher line against Tehran.

Iranian President Mahmoud Ahmadinejad – who this week branded Mr Obama a nuclear-armed "cowboy" – was defiant, insisting Iran would "try to make an opportunity out of sanctions" rather than change its atomic policy to avoid them.

"We do not welcome the idea of threat or sanctions, but we would never implore those who threaten us with sanctions to reverse their sanctions against us," an official Iranian news agency quoted him as saying.

Russia has considerable influence over Iran, having built a new nuclear reactor at the port of Bushehr, and the Obama administration has sought to coax Moscow closer to its position on Tehran's nuclear ambitions during talks on the disarmament treaty.

The strategic limitation pact was signed almost exactly a year after the newly elected Mr Obama delivered a speech to a crowd of 20,000 people in central Prague in which he set out a bold vision of a world without nuclear weapons. Central and eastern European states have expressed concerns that their interests could be sacrificed in America's bid to "reset" relations with Russia. But Mr Obama was expected to address those worries last night at a US embassy dinner in Prague with 11 of the region's leaders.

Prospects for ratification of the treaty are not completely smooth. Both the US Senate and the Russian parliament have to rubber stamp the accord. The Duma is expected to follow the lead of the Kremlin but achieving the required votes in the Senate will be more difficult. Mr Obama said yesterday he hoped it would be ratified by the end of the year although this will require Republican support.

* Iran has accused three jailed Americans of links to US intelligence services, state television said yesterday. The three were held on suspicion of spying after crossing the border from Iraq in July. Intelligence minister Heidar Moslehi said there was "credible evidence that they have links to American intelligence" state-run Press TV reported.

Nuclear weapons by numbers

2,200 The number of strategic warheads the US and Russia have been allowed since 2002

1,550 The number allowed under the new treaty

32,193 US nuclear weapons stockpile at its peak

30,000 Number of nuclear weapons believed to exist in the world today

13 kilotons The blast yield of the atomic bomb dropped on Hiroshima

10-15 megatons The largest nuclear weapon ever deployed