The Iranian military staged missile exercises today in a move that is sure to further inflame an international row over the country's nuclear capability.
The tests involved short-range missiles fired during drills by the regime's Revolutionary Guards, according to Iranian state television.
David Miliband warned that the Middle Eastern regime must take "concrete steps" to allay fears that it is building a nuclear arsenal.
The Foreign Secretary insisted that the focus remained on a diplomatic solution after revelations that Iran is building another nuclear facility.
But he repeatedly declined invitations to describe military intervention as "inconceivable".
Iranian leader Mahmoud Ahmadinejad is under pressure after the existence of the covert site, buried in a mountainside near the holy city of Qom, was revealed.
Mark Fitzpatrick from the International Institute for Strategic Studies said Tehran could have nuclear weapons in as little as a year.
He said: "If they decided today to go for a nuclear weapon and they didn't care about anybody knowing about it, it's possible they could do it in a year. Probably longer, but if all the steps went like clockwork then maybe a year.
"It's likely that they have some secret facilities and how far along they are in those facilities is a guess.
"If they were to develop a nuclear weapon they would probably do it at a clandestine facility so that they wouldn't trigger the obvious trip wire."
Iranian officials will meet representatives of the E3+3 group of Britain, France, Germany, the US, Russia and China, in Geneva next Thursday.
Questioned about the likelihood of military force against Iran, Mr Miliband said: "No sane person looks at the military question of engagement with Iran with anything other than real concern.
"That's why we always say we are 100% committed to the diplomatic track."
But, questioned on BBC Radio 4's Today programme yesterday, Mr Miliband declined to describe military action as "inconceivable" - the word used by Jack Straw when he was foreign secretary.
"I always say to people look at what I do say, not at what I don't say and what I do say is that we are 100% focused on a diplomatic resolution of this question," Mr Miliband said.
"It's vital that we remain so, it's vital that in the very short term in a meeting next Thursday that the Iranians take practical and concrete steps to address the outstanding questions."
Mr Ahmadinejad said that the new facility would not be operational for 18 months so he had not violated any requirements.
He maintained that Iran opposes nuclear weapons as "inhumane".