BBC News
The foreign secretary said Iran had a "once in a generation" chance to normalise relations with the US.
The Obama administration says it will work with Iran, whose nuclear programme has been the source of longstanding tension, if it "unclenches its fist".
Gordon Brown has said Iran faces a "clear choice" over its nuclear plans.
Backing US approach
Earlier this month, he said the international community would help Iran develop a civil nuclear capacity as long as it brought its nuclear facilities under international supervision and complied within UN regulations.
Tehran says its nuclear programme is entirely peaceful but it has continued to enrich uranium in defiance of international pressure, leading many to conclude it hopes to develop a nuclear bomb.
The UN has already approved three rounds of sanctions against Iran and the US has warned of further punitive measures if Iran does not take up its offer of a "new beginning" in relations.
But during a debate in Parliament, Mr Miliband said "now is not the time to be rushing for more sanctions" against Iran.
"Now is the time to be backing the American outreach that is a once-in-a-generation opportunity not just for us but for the Iranians," he told MPs.
Iran has given a guarded response to the more conciliatory tone coming out of Washington.
Officials from the two countries met in the Hague on Tuesday to discuss joint efforts to promote greater stability in Afghanistan.
