The United States heralded a potential breakthrough on Wednesday in building support for UN sanctions against Iran, saying China was now ready for "serious" talks at the United Nations.
"China has agreed to sit down and begin serious negotiations here in New York," the US ambassador to the United Nations, Susan Rice, said on CNN television.
"This is progress, but the negotiations have yet to begin in earnest," Rice said.
US President Barack Obama on Tuesday said he wants sanctions agreed on within weeks and Rice said Washington was airing "tough" proposals.
She said the talks would take place between the six big powers coordinating a response to Iran's controversial push for what it says is a civilian nuclear capacity, but which Washington says could mask a military program.
China has veto power on UN Security Council resolutions and has consistently opposed punishing Iran.
Secretary of State Hillary Clinton was upbeat, saying the P5+1 group -- permanent council members Britain, China, France, Russia and the United States, plus Germany -- "continues to be unified."
"There will be a great deal of further consultation, not only among the P5+1, but with other members of the Security Council and other nations during next weeks," Clinton said.
Earlier Wednesday, a State Department spokesman said that senior diplomats from the six powers, including China, had spoken by conference call on "consultations on next steps."
"We're in a period of intense diplomatic engagement on this issue and this call was within that context," said spokesman Mark Toner.
That discussion came about one week after a similar conference call in which China participated after weeks of stalling.
The Security Council already has slapped three rounds of sanctions on the Islamic republic over its refusal to halt uranium enrichment, which the West and Israel view as a cover to build nuclear weapons.
Iran denies the charges and maintains that its nuclear program is solely geared toward electricity generation for its growing population.
Iran's chief nuclear negotiator, Saeed Jalili, is to visit China on Thursday, Iran's official IRNA news agency said.
He was planning to meet high-ranking officials in Beijing.
China's position remains key to the future of the long-running stand-off.
"The point is we (the US and China) have very different perspectives," Dean Cheng, from the Heritage Foundation, a conservative think-tank, told AFP, recalling that Iran is one of Beijing's main oil suppliers.
Karim Sadjadpour, an expert from the Carnegie Institute, said China would probably end up compromising by agreeing to sanctions -- but in a watered down version to what Washington wants.
"Though China has been dragging its feet, I think they will eventually meet Washington halfway," Sadjadpour said.
Other members of the Security Council -- Brazil, Lebanon and Turkey -- remain opposed to sanctions. Although they are non-permanent members and have no right of veto, their votes will count when it comes to approving a resolution.
Brazil's Foreign Minister Celso Amorim, in New York to attend a conference on Haiti, reiterated his country's position on the Iran nuclear issue, saying "Brazil is in favor of a negotiated solution."
China's support for sanctions would be crucial in winning over the final doubters and dissuading them from voting against the resolution, Sadjadpour said.
Washington is also hoping to benefit from Japan's presidency of the Security Council in April to be able to bring a resolution to the table.
The spokesman for the Japanese foreign ministry Kazuo Kodama has stressed for his part that Tokyo will take seriously any chance to boost international security.