Mr Brown said unilateral action by the UK was "not the best way"
Cuts in the UK's Trident nuclear deterrent could be considered as part of non-proliferation talks called by the US, Gordon Brown has said.
The G8 has backed US President Barack Obama's plan for a nuclear proliferation summit in spring 2010.
The prime minister made clear the government's position on Trident had not changed and he ruled out unilateral cuts in either submarines or warheads.
But he said nuclear powers should cut their arsenals by "collective action".
Any potential deal would involve nuclear powers reducing their stockpiles and offering civilian nuclear know-how to non-nuclear states in return for them promising not to develop nuclear weapons.
Speaking at the G8, the prime minister made clear the government's position on maintaining Trident had not changed and he ruled out any unilateral cuts in either submarines or Britain's 160 warheads.
Mr Brown said that he will set out within the next few days specific recommendations for a future non-proliferation framework, which could see nuclear-armed states reduce their own arsenals and offer help with the development of civil atomic energy capacities, in return for non-nuclear states promising not to try to get the bomb.
But he added: "Iran is attempting to build a nuclear weapon. North Korea is attempting to build a nuclear weapon. We have got to show we can deal with this by collective action.
"Unilateral action by the United Kingdom would not be seen as the best way. What we need is collective action by the nuclear weapons powers to say that we are prepared to reduce our nuclear weapons, but we need assurances also that other countries will not proliferate them.
"And we need new kinds of assurances to prevent a situation such as we have got in Iran emerging in exactly the same way again."
Mr Brown made similar suggestions in a speech in March.
But BBC chief political correspondent James Landale said his latest comments have added urgency and significance following the announcement of the non-proliferation talks next spring in Washington, ahead of a review of the Non-Proliferation Treaty.
Mr Brown is also coming under increasing pressure from Labour backbenchers to scale back his planned multi-billion investment in Trident to help deal with Britain's escalating debts.
Welcoming the agreement on a spring conference, Mr Brown said: "There is a possibility of a nuclear deal that we will help countries that are non-nuclear gain access to civil nuclear power and to do it in a way that is safe for the whole of the world, but we want them to agree to tight conditions about non-proliferation of nuclear weapons.
"We will try at the same time to talk with Russia and America to achieve some reduction in nuclear weapons."
Mr Brown said the proposed new requirement for non-nuclear states to prove they do not have a military capability was not a matter of treating countries like Iran as "guilty until proven innocent", but was a recognition that states signing up to non-proliferation agreements ought to be able to show that they are complying with them.