France's President Nicolas Sarkozy warned on Wednesday that Iran's alleged attempts to build long-range missiles and nuclear weapons could lead unnamed countries to launch a pre-emptive attack.

"Its military nuclear and ballistic ambitions constitute a growing threat that may lead to a preventive attack against Iranian sites that would provoke a major crisis that France wants to avoid at all costs," he said.

Sarkozy did not say which country might launch such a strike, but it has been reported that Israel -- perhaps with US support -- has considered bombing Iranian nuclear sites if it believes Tehran is close to building a weapon.

The French leader placed the blame for the crisis on Iran, which insists it has no intention of building a nuclear weapon, and is merely enriching nuclear fuel for medicial research and a domestic atomic energy programme.

"Iran refuses to negotiate seriously," he told an annual meeting of French diplomats. "Iran is carrying out new provocations in response to the challenge from the international community for it to provide a credible response."

Sarkozy said France would work with its allies to build support for tougher international sanctions against Tehran's Islamist regime, in a bid to force it to back down over its enrichment programme.

Tehran currently does its uranium enrichment, the most sensitive part of its programme, at the Natanz facility in central Iran, with plans to divert the 20-percent purification process to a new site near the holy city of Qom.

The UN Security Council has repeatedly ordered Tehran to halt all uranium enrichment until its agency the International Atomic Energy Agency (IAEA) is satisfied by of the peaceful nature of its nuclear activities.

But, despite being targeted by four sets of UN Security Council sanctions over its refusal to suspend enrichment, Iran remains adamant that it will push ahead and denies Western claims that it seeks to build a nuclear bomb.

This week Iran upped the ante once again, announcing that it had abandoned talks with the international community to negotiate a nuclear fuel swap that would see it forego its own enrichment in return for civilian-level fuel.

Western powers fear that if Iran perfects enrichment technology it could rapidly convert uranium into weapons grade material and thus be considered a "threshold" nuclear power capable of rapidly constructing a bomb.