The Canadian delegation walks out as Iranian President Mahmoud Ahmadinejad (2nd-R) prepares to address the 64th session of the United Nations General Assembly on Wednesday at the United Nations in New York. Photo: AFP

World powers leading talks to rein in Iran's suspect nuclear ambitions upped the pressure on the Islamic republic on Wednesday, as momentum grew toward imposing new sanctions.

As Russian President Dmitry Medvedev appeared to make a major policy shift by countenancing tough new nuclear sanctions, the global powers said they still had "serious concerns" about Tehran's nuclear programme.

But China on Thursday reiterated its opposition to sanctions on Iran over its nuclear programmeme, as Tehran came under mounting pressure from world leaders at the United Nations to stop uranium enrichment.

"We always believe that sanctions and pressure are not the way out," foreign ministry spokeswoman Jiang Yu told reporters.

Top diplomats from the five permanent members of the United Nations Security Council -- the United States, Russia, China, Britain and France -- plus Germany called on Iran to meet its international obligations as they prepared for talks next week in Geneva.

"Iran's nuclear programme remains a matter of serious concern to the international community," British Foreign Secretary David Miliband said in reading a statement from the six powers.

"We acknowledge the recent measures taken by Iran regarding its cooperation with the IAEA and encourage Iran to cooperate further with the IAEA to resolve the remaining issues which need to be clarified to exclude the possibility of military dimensions to Iran's nuclear programme."

He called on Iran to implement all the demands of the International Atomic Energy Agency and the UN Security Council "to build confidence in the exclusively peaceful nature of its nuclear programme at the earliest possible date."

He said the group, known as the P5 + 1, expects "a serious response from Iran" on October 1.

Russia meanwhile moved closer than ever before to Washington's position that the Islamic Republic should face crippling sanctions unless it makes concessions on its nuclear programme in the talks.

"Sanctions rarely lead to productive results, but in some cases sanctions are inevitable," Medvedev said, after talks with US President Barack Obama on the sidelines of the UN General Assembly in New York.

The apparent US and Russian convergence came as Iranian President Mahmoud Ahmadinejad addressed the annual debate.

Obama said after the talks at the Waldorf Astoria hotel in New York that time was running out for Iran to open the doors of its nuclear programme to the rest of the world.

"Serious additional sanctions remain a possibility," the president said, calling on Tehran to "seize the opportunity" at the talks with the P5 + 1.

While stiffening his position, Medvedev did call on the international community to send the right signals and incentives to Iran to convince it to halt what Western powers and Israel claim is a cover to produce nuclear weapons.

"We need to help Iran to (make) the right decisions," he said.

US officials welcomed the remarks.

"We are in a different place in US-Russia relations, said Michael McFaul, the top official responsible for Russia on Obama's National Security Council, adding there was no "daylight" between the sides on the Iran issue.

"I cannot improve on what President Medvedev said. He could not have been clearer."

Taking a more conciliatory tone than his predecessor George W Bush, Obama has asked Iran to accept an outstretched hand of friendship and engage with the United States on nuclear and other issues.

But on Wednesday, Obama urged world leaders to hold Iran -- as well as North Korea -- to account for their nuclear programmes, warning they threatened to take the world down a "dangerous slope.”