Gordon Brown has refused to rule out military action against Iran over its nuclear plans, but said he would prefer to resolve it through diplomacy.
Mr Brown said he would not rule anything out
He said Iran - suspected of enriching uranium to use in weapons - was breaching international treaties.
But the prime minister said he thought Tehran could be persuaded to meet its obligations through diplomacy - sanctions if necessary.
Iran says it wants nuclear power for civilian use - not to build a weapon.
But the UN Security Council has imposed sanctions on Iran because of its refusal to suspend all uranium enrichment activity.
In his regular press conference on Monday, Mr Brown was asked if he would confirm he would neither "support nor assist" a US attack on Iran.
He said the matter could not go unchallenged but added: "We believe that this matter can be resolved by diplomatic means, by the resolutions that have been passed by the United Nations, by sanctions if necessary.
"But I'm not prepared to go further than that. What I am prepared to say is that we take very seriously what Iran is proposing and we are prepared to use the methods that we have used in diplomatic sanctions to deal with this problem - and I do not rule out anything."
On Friday Iranian President Mahmoud Ahmadinejad made a defiant speech defending Iran's nuclear programme.
Mr Ahmadinejad said: "The Iranian people are not ready to sit around a table and discuss their absolute nuclear rights. They [the world powers] have to know this."