The US accused Iran on Thursday of "stringing along" UN watchdogs investigating its nuclear ambitions and bluntly warned China not to block new sanctions against the Islamic republic.
US officials talked tough after the UN's International Atomic Energy Agency (IAEA) said Iran had made "substantial progress" but that if needed to be more pro-active in shedding light on its program.
One official close to the IAEA said the assessment showed that "the glass is half full" with regard to the Iranian case.
But White House spokeswoman Dana Perino said Tehran was guilty of "selective cooperation."
"This report sadly makes clear that Iran seems uninterested in working with the rest of the world," she said.
"The United States will work with our partners on the UN Security Council and Germany as we move towards a third set of Security Council sanctions," Perino said.
At the UN, US envoy Zalmay Khalilzad accused China of stalling on sanctions designed to punish Iran's refusal to halt uranium enrichment.
Enriched uranium can be used to generate nuclear power, but also in a highly refined form, to make nuclear weapons.
"I don't think China would want to be in a position to cause a failure of diplomacy to deal with this issue," Khalilzad told reporters.
"For diplomacy to succeed it needs widely supported, broad and biting sanctions to affect the calculations of the regime in Iran," he said.
The US has never ruled out military action if Iran persists with its atomic drive, and has stepped up its rhetoric of late to warn of "World War III" if Tehran gains the know-how to begin manufacturing nuclear weapons.
A senior UN official confirmed a claim by Iranian President Mahmoud Ahmadinejad that Iran had reached a key target of 3,000 centrifuges enriching uranium.
Scientists say that number of centrifuges could be sufficient to produce enough enriched uranium in one year to manufacture a single nuclear bomb.
But Washington has been frustrated at the UN Security Council by the reluctance of China in particular to intensify two earlier rounds of global sanctions against Iran.
"It's in everyone's interest for this world-defining issue to be resolved diplomatically," the US envoy said.
The IAEA, whose chief Mohamed ElBaradei has strongly criticized the US approach to Iran, said the country had given "sufficient access" to its inspectors and had responded "in a timely manner" to questions.
"However, its cooperation has been reactive rather than pro-active," the report said, stressing that Iran's "active cooperation and full transparency" were needed to assuage international concerns.
At the White House, Perino zeroed in on what she described as the report's finding that "contrary to the decisions of the Security Council, Iran has not suspended its enrichment-related activities."
"We believe that Iran should be fully cooperating, and not stringing along the IAEA during this process," she said. "It just pushes Iran further into isolation and we believe that what they should do is take us up on our offer."
The US, Britain, France and Germany have offered a package of political and economic incentives to Iran, including cooperation on civil nuclear energy, if the country freezes enrichment.
Washington and its allies are also awaiting a report from EU foreign policy chief Javier Solana on his own contacts with Iran.