A surprise, high-profile UN dinner failed to break the deadlock with Iran over its nuclear plans as the United States called it a "missed opportunity" and kept up the pressure Friday for UN sanctions.

The Obama administration also claimed that Iran's dinner invitation to all 15 UN Security Council members on Thursday is another sign that Tehran is worried about its international isolation and that US diplomacy is paying off.

State Department spokesman Philip Crowley said the US and other guests at the dinner hosted by Iranian Foreign Minister Manouchehr Mottaki failed to bridge gaps over a proposed nuclear fuel swap deal with Iran.

In a "frank and professional exchange" with Mottaki, US diplomat Alejandro Wolff and other council representatives "pointed out the significant flaws and shortcomings in Iran's approach," Crowley said.

"Mottaki focused on the Iranian counterproposal to the Tehran research reactor, which deviates in significant ways from the balanced IAEA proposal that Iran agreed to and then walked away from last October," Crowley said.

"But we see this as yet another missed opportunity by Iran to meet its international obligations," Crowley said.

In a bid to boost trust, the UN International Atomic Energy Agency (IAEA) proposed last year that Iran send most of its lower-grade uranium abroad to be further enriched and sent back for medical research purposes.

The United States has been spearheading a drive for a fourth round of UN Security Council sanctions. It is trying to get Iran to stop enriching uranium, which can be used as fuel either for civilian power reactors or atomic weapons.

However, China is the main holdout to tougher sanctions on the UN Security Council, along with Brazil, Turkey and Lebanon. A previously reluctant Russia now appears more open to sanctions.

But Crowley said that, during the dinner, both Russia and China joined "in pressing Iran... to change its course."

China and Russia along with Britain, France and the United States are the permanent five veto-wielding members of the Security Council.

Crowley also said US Secretary of State Hillary Clinton meanwhile Friday reviewed efforts for a new sanctions resolution in a conference call with senior diplomats from France, Britain, Germany and the European Union.

"There's still work (for a resolution) to do. And we will... be moving that forward in the coming weeks," Crowley said.

Anne-Marie Slaughter, the State Department's director of policy planning, told department colleagues that Iran is trying all the harder to engage the international community in a bid to stop its growing isolation.

"President (Mahmoud) Ahmadinejad coming to the UN, the dinner last night... I read these as signs that the government is quite worried," Slaughter told diplomats and Foreign Service staff in a speech broadcast to journalists.

The Iranians are trying harder than in the past "to try to stop anything that will make them more isolated. I read that as some sign of our success," Slaughter said.

Ahmadinejad was the only head of state to travel to the United Nations for the first two days of the nuclear Non-Proliferation Treaty review conference.

And the dinner invitation to the 15 council members -- with journalists observing all but diplomats from Nigeria and Gabon showing up -- yielded one of the highest-level US-Iran contacts since the 1979 Islamic Revolution.

Washington and Tehran have had no diplomatic relations since April 1980, and face-to-face encounters between the countries' senior officials are rare.

Meanwhile, Wolff, the deputy permanent US representative to the UN, renewed US calls for the release of all Americans held in Iran when he chatted with Mottaki Thursday evening, Crowley said.

US hikers Shane Bauer, Sarah Shourd and Josh Fattal were arrested last July 31 after straying across the border during a hiking trip in Iraq's Kurdistan region.

Iranian Intelligence Minister Heydar Moslehi charged in early April that the three were working with intelligence services.

Washington also wants the release of US citizens Reza Taghavi and Kian Tajbakhsh, and remains concerned about the fate of Robert Levinson, a former FBI agent who went missing during a visit to Iran in 2007.