Russia toughened its stance on Iran's nuclear project on Tuesday after the Islamic state ramped up uranium enrichment, and US officials said a new UN sanctions resolution could be ready in weeks.

The powerful head of Russia's national security council, Nikolai Patrushev, said Tehran's announcement that it has started work to produce 20 percent enriched uranium cast doubt on its claims not to be pursuing atomic weapons.

Russia has traditionally resisted threats of tough action against Iran over fears it is developing a nuclear bomb, but Patrushev indicated the Kremlin's patience in trying to seek dialogue with Tehran was wearing out.

"Iran claims it is not trying to acquire nuclear weapons," Patrushev was quoted as saying by Russian state news agencies.

"But actions such as starting to enrich low-enriched uranium up to 20 percent raise doubts in other countries and these doubts are fairly well-grounded," Patrushev added.

"Political and diplomatic methods are important for regulating, but everything has its limit," Patrushev was quoted as saying.

His comments were an unusual expression of concern from Moscow, which has long said there was no evidence that Iran was pursuing anything other than a civilian nuclear energy programme.

The United States and France have already pressed calls for "strong" new sanctions against Iran, leaving China as the main power holding out, and Beijing again called for talks on a nuclear fuel deal with Iran.

"We hope the relevant parties will exchange views on the draft deal on the Tehran research reactor and reach common ground at an early date which will help solve the issue," foreign ministry spokesman Ma Zhaoxu said.

The foreign minister of Turkey, a country with good ties with both Iran and the West, is to visit Iran next week to push for a diplomatic solution to the stand-off.

Ahmet Davutoglu, speaking in Ankara, insisted a deal for enriching uranium for Iran, proposed by the international group, could still offer a peaceful solution.

Washington has called the latest Iranian moves "provocative" and is pressing ahead with moves at the UN Security Council for a fourth sanctions resolution against Iran.

US Defence Secretary Robert Gates "thinks this is a matter of weeks, not months," Pentagon spokesman Geoff Morrell told reporters in Paris, where Gates met French President Nicolas Sarkozy.

"He thinks that we need it and that we can do it in that time," Morrell added. "In all his meetings he discussed this sense of urgency."

The loudest call for sanctions came from Israel where Prime Minister Benjamin Netanyahu called for "crippling" measures against Tehran.

At a gathering of European Union ambassadors in Jerusalem, Netanyahu said Iran was "racing" to produce nuclear weapons in defiance of the international community and strong action was vital.

"I believe that what is required right now is tough action from the international community. This means not moderate sanctions or watered-down sanctions; this means crippling sanctions and these sanctions must be applied right now," he said.

In a further sign of rising tensions over the nuclear issue, around 100 pro-government militia members tried to storm Italy's embassy in Tehran, Italian Foreign Minister Franco Frattini told a Senate hearing.

Similar protests took place outside the French and Dutch missions in Tehran, Frattini said.

On Monday Sarkozy and Gates agreed to push for "strong" new anti-nuclear sanctions against Iran, which insists its atomic drive is to produce energy.

The UN Security Council has already passed three sanctions resolutions against Iran, and Gates told US news channel Fox on Monday a new one would provide a legal platform to "perhaps take even more far-reaching" action against Iran.

France holds the presidency of the council this month, but its Foreign Minister Bernard Kouchner said Monday that China's opposition could prevent any resolution being passed during February.

burs-pdw/co