The US and its allies will have to push for "punitive" measures on Iran after it failed to respond positively to an incentives offer, the White House says.
The five permanent members of the UN Security Council and Germany are due to hold telephone talks about Iran's nuclear programme, BBC reported.
Iran sent a letter on Tuesday saying it was ready to give a "clear response" to the offer if it got the same in return.
White House spokeswoman Dana Perino said the Iranians had been stalling.
On Tuesday, the US state department said Tehran's letter, delivered to the EU's foreign policy chief, did not give a clear message and that "additional measures" would be unavoidable.
The telephone conference between the six nations - China, France, Russia, the UK and US, along with Germany - was due to take place at 1115 GMT on Wednesday.
The move follows "inconclusive" talks between the EU and Tehran over Iran's nuclear enrichment programme.
The Iranian government insists its nuclear programme is for entirely peaceful purposes, while the US and its allies believe it could be used to develop nuclear weapons.
The group of six world powers tabled the latest proposal to Iran in June in an effort to persuade it to suspend uranium enrichment as an initial step in starting talks on a longer-term deal.
EU foreign policy chief Javier Solana had given Iran's chief nuclear negotiator until last Saturday to respond to an offer of no further economic sanctions in return for an Iranian freeze on uranium enrichment.
Speaking to reporters from a plane carrying US President George W Bush on an Asian tour, Ms Perino said the Iranians had failed to provide "a decent and responsive statement" to the offer.
"In the absence of a positive response... we think that the allies will have no choice but to take further measures that would be punitive," she said.
US media reports have said the US intends to push for new sanctions against Iran at the UN Security Council.
Meanwhile, the UN's nuclear watchdog, the International Atomic Energy Agency (IAEA), said its deputy director general, Olli Heinonen, would visit Tehran for talks on Thursday.