Washington will insist on referring Iran to the United Nations Security Council over its nuclear programme, the US Secretary of State has said.
Iran says it wants the technology for energy purposes alone
Condoleezza Rice told Reuters news agency that "Iranians are feeling the heat" of international pressure.
She said a compromise Russian plan to resolve the stand-off was merely a delaying tactic by Iran ahead of a UN's nuclear watchdog meeting next month.
Tehran denies US claims that it is seeking to develop nuclear weapons.
The Iranians are "doing nothing but trying to throw up chaff so that they are not referred to the (UN) Security Council and people shouldn't let them get away with it," Ms Rice said.
IRAN'S NUCLEAR STAND-OFF
Sept 2002: Work begins on Iran's first reactor at Bushehr
Dec 2002: Satellites reveal Arak and Natanz sites, triggering IAEA inspections
Nov 2003: Iran suspends uranium enrichment and allows tougher inspections
June 2004: IAEA rebukes Iran for not fully co-operating
Nov 2004: Iran suspends enrichment under deal with EU
Aug 2005: Iran rejects EU plan and re-opens Isfahan plant
Jan 2006: Iran re-opens Natanz facility
"The time for talking outside the Security Council is over," she added.
The UN's nuclear watchdog, the International Atomic Energy Agency (IAEA), is meeting in Vienna on 2 February to ponder sending Iran to the UN Security Council.
It is thought to be split over whether to refer Iran to the Security Council, and UN Secretary General Kofi Annan said the IAEA's latest report on Iran might not be ready until March.
Moscow has offered that instead of a formal referral for possible sanctions, the nuclear watchdog could ask the Security Council to discuss the issue and then send it back to the IAEA.
The Russian proposal would limit Iran's ability to covertly produce fissile material for possible use in a nuclear weapon by carrying out uranium enrichment in Russia.
China, which opposes sanctions against Iran at this stage and has the right to veto them at the UN Security Council, has also welcomed the Russian proposal.