The Iranian military has staged missile exercises 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."