President Mahmoud Ahmadinejad at Iran's first nuclear plant last year
Foreign Secretary David Miliband has said "no sane person" would look to attack Iran over its nuclear program "without real concern".
Interviewed on the BBC, he said the UK was "100% committed" to finding a diplomatic solution.
But Mr Miliband refused to rule out military action altogether, declining to say whether it was "inconceivable".
Iran says a second plant will soon be operational but denies it is trying to make nuclear weapons.
Iranian nuclear officials meet on Thursday with the permanent members of the United Nations Security Council plus Germany in Geneva.
Israel's foreign minister Avigdor Lieberman has called for an "unequivocal" response from Western leaders.
"The revelations of this second nuclear enrichment site in Iran prove beyond any doubt that this country wants to equip itself with nuclear weapons", he told Israeli public radio.
Meanwhile, US President Barack Obama kept up the pressure on Iran in his weekend radio and internet address.
He said Iran would "face increased pressure and isolation" unless Tehran cooperated "fully" with UN nuclear inspectors.
Mr Miliband told BBC Radio 4's Today programme that the "dual track strategy" of diplomacy and pressure on Iran would depend on the outcome of the meeting.
BBC World Affairs correspondent Paul Reynolds
Iranian ambitions for this site are not known. It could be that they wanted a back-up in case their main plant at Natanz was attacked. But another fear is that they intended to enrich uranium more highly at the secret plant, to a level suitable for a nuclear explosion.
The discovery will strengthen the demands by the US and its allies for further sanctions to be imposed on Iran unless it suspends all enrichment, as required by the Security Council.
He said: "I think it's very important we stick to that because the diplomatic track of engagement on the one hand and pressure on the other is only now really being tried with the engagement of America."
But pressed on whether military action was "inconceivable", he added: "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."
President Mahmoud Ahmadinejad of Iran says his country is not interested in developing nuclear weapons, describing them as "against humanity".
Leaders of the US, UK and France have accused Iran of building the new plant to enrich uranium, in breach of UN rules.
They have threatened Iran with new, tougher sanctions if it does not fully co-operate with the global powers.
Iran has denied claims - made earlier by Mr Obama, French President Nicolas Sarkozy and Gordon Brown - that the second plant near the city of Qom was a secret facility.
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