An Iranian offer to negotiate on its nuclear programme falls short of UN demands, the US has said.
The incentives package set a deadline of 31 August for research to halt
Iran had offered "serious talks" in response to a package of incentives offered if it halted uranium enrichment by 31 August.
But the US said suspension of research was required first, echoing French comments. China and Russia said earlier that talks were the only way forward.
Iran could face sanctions, amid claims it is making a bomb - which it denies.
The US, UK, Russia, China, France and Germany offered Iran the package of incentives - including help with civilian nuclear technology - in exchange for suspending enrichment.
Iran's chief nuclear negotiator, Ali Larijani, said on Tuesday that his country was ready for "serious talks" on the issue - but did not give any more details.
In Washington, a state department spokesman said the US was consulting closely, including with other members of the Security Council, on the next steps.
"We acknowledge that Iran considers its response as a serious offer, and we will review it," said Gonzalo Gallegos.
Iran says its nuclear programme is for power generation only
"The response, however, falls short of the conditions set by the Security Council, which require the full and verifiable suspension of all enrichment-related and reprocessing activities."
French Foreign Minister Philippe Douste-Blazy also said fresh talks were dependent on Tehran halting its research.
"The Iranians know the rules of the game: first a suspension of sensitive nuclear activities," he said.
Beijing said it was "carefully studying" Iran's reply.
"China has always believed that seeking a peaceful resolution of the Iranian nuclear issue through diplomatic talks is the best choice and in the interests of all parties concerned," the Chinese foreign ministry said.
Russia echoed the Chinese stance, stressing its commitment to a negotiated solution to the crisis.
BBC state department correspondent Jonathan Beale describes the US response as a carefully worded statement, in that it did not at first appear to dismiss Tehran's offer of "serious talks" completely out of hand.
But the US promise to "review" the offer is more likely to be aimed at reassuring Russia and China than Iran, he says - the US still needs to persuade all members of the UN Security Council that it has been willing to negotiate.
But in the end the US has now made its position clear by stating that Iran's latest offer is still not enough - and that the US is now considering "next steps" - in other words the threat of sanctions, our correspondent adds.
Iran points out that as a signatory to the nuclear Non-Proliferation Treaty (NPT) it is entitled to a nuclear power programme and says it has broken no rule.
But the Western powers accuse Iran of concealing an enrichment programme, and Washington has refused to rule out military action.
Meanwhile, a report by UK-based think tank Chatham House says Iran can afford to continue equivocating in the dispute because of its regional supremacy.
The report argues that Iran has emerged as the biggest beneficiary of the US-led "war on terror" in the Middle East.
Recent US-led wars have "eliminated two of Iran's regional rival governments - the Taleban in Afghanistan and the Saddam Hussein regime in Iraq in April 2003".