Tehran says its nuclear programme is peaceful
Iran's top nuclear official is holding talks in China amid signs Beijing may have relaxed its opposition to a new round of UN sanctions against Tehran.
The US ambassador to the UN, Susan Rice, said China was ready to hold "serious" talks with Western powers on a new UN resolution.
This would mark a policy shift for China, which has strong ties with Iran. China says it wants a peaceful outcome.
Western powers claim Iran seeks nuclear weapons, a charge Tehran denies.
China has in the past expressed reluctance to see new sanctions imposed, and as a permanent member of the UN Security Council, along with France, the US, the UK, and Russia, it has the power to veto such a move.
US President Barack Obama, meanwhile, has said he hopes to have sanctions in place "within weeks".
'Tough sanctions regime'
Paul Reynolds, BBC News world affairs correspondent
Until now China has stressed the need for further talks with Iran - but even China seems to accept these have not got anywhere.
Chinese support for a new round of Security Council-led sanctions on Iran would increase the diplomatic and economic pressure on Iran, but would probably come with a price tag.
China has a veto in the council and, if it agrees to new sanctions, it is likely to say these should not be as severe as some would want.
China has substantial contracts with Iran for the supply of oil and gas, so targeting the energy sector is out.
Likewise, China might be reluctant to disturb Iran's international trade, by imposing restrictions on re-insurance.
The most probable target is the business portfolio of the Iranian Revolutionary Guard.
Making an arms embargo on Iran mandatory is also a possibility.
Beijing made the commitment to talks in a phone call on Wednesday, US officials told the Associated Press news agency.
Ms Rice said the US, the UK, France, Germany, Russia and China had agreed to begin drafting a UN resolution on Iran's nuclear programme.
She told CNN: "China has agreed to sit down and begin serious negotiations here in New York with the others in the [group of six] ... as a first step towards getting the entire Security Council on board with a tough sanctions regime against Iran.
"This is progress, but the negotiations have yet to begin in earnest."
One group of six leader, German Chancellor Angela Merkel, had also spoken to Chinese officials, holding phone talks with Chinese Premier Wen Jiabao, AFP news agency reported.
Chinese Foreign Ministry spokesman Qin Gang said President Hu Jintao would attend a summit on nuclear security in Washington later in the month.
Mr Qin did not comment directly on Ms Rice's words but, in response to a question about them, said: "We have always and will continue to push for a peaceful settlement of this issue."
He added the standoff should be resolved by "diplomatic means".
UN SANCTIONS AGAINST IRAN
Resolution 1737 (Dec 2006) - Ban on supply of materials and technology which could contribute to uranium enrichment or making of nuclear weapons delivery systems
Resolution 1747 (Mar 07) - Prevents dealings with Sepah bank and 28 named people and organisations, bans arms imports from Iran and limits loans to humanitarian and developmental purposes
Resolution 1803 (Mar 08) - Asset restrictions, travel bans extended, sale of dual-use items banned, governments called on to withdraw financial backing for companies trading with Iran
"China strongly values the nuclear security issue and opposes nuclear proliferation and nuclear terrorism," he said.
Meanwhile Iran's chief nuclear negotiator Saeed Jalili arrived in Beijing for "bilateral talks and to discuss the nuclear issue", Iran's Irna news agency reported.
"The relationship between Iran and China is very important, and it is very important for our two countries to cooperate on all the issues," he told Iranian media.
On Wednesday, US media reported an Iranian nuclear scientist had defected.
ABC News said Shahram Amiri had been resettled in the US and was helping the CIA try to block Iran's nuclear programme.
Iran accused the US of abducting him, but Washington has denied any knowledge of the scientist.