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BRUSSELS, Oct 3 (Reuters) - French Foreign Minister Bernard Kouchner has urged European Union counterparts to study widening existing sanctions on Iran's banking sector over its nuclear programme before any new U.N. resolution against Tehran.

"These new measures, coming from its most important commercial partner, should have the aim of increasing the pressure on Iran, in particular in the financial and economic area," Kouchner wrote in a letter to fellow EU ministers, a copy of which was obtained by Reuters on Wednesday.

"Initially, we could add new entities, in particular in the banking sector, and new individuals to the existing European lists of asset freezes and visa bans," he added, urging a debate on such measures at an Oct. 15 meeting of EU foreign ministers.

However Italian Prime Minister Romano Prodi said separately that any sanctions should be decided in the United Nations and he did not favour a tightening of measures at the moment.

"The doctrine of Italy is that there is one place where sanctions are decided, that place is called the United Nations. We stand by that doctrine," he told a news conference in Rome.

"We must keep those sanctions in the background but I don't think to stiffen them before sitting at the negotiating table would be the most accurate choice," he said.

Kouchner stressed the EU should continue diplomatic efforts to encourage Tehran to suspend its uranium enrichment activities and that the priority remained securing a tough U.N. Security Council resolution.

"But as I was able to ascertain at the meeting that took place in New York on Sept. 28 ... that negotiation will take some more time yet because of the positions of some of our partners," he said.

"France is ready to make concrete proposals in this respect," he added.

Major world powers last week delayed a vote on tougher sanctions on Iran until late November at the earliest, with U.N. Security Council veto-holder Russia stresing the need to see if there could be a breakthrough in negotiations with Tehran.

Britain backs Kouchner's push for tougher sanctions but aside from Italy, a major commercial partner of Tehran, Germany is known to be more reticent.

Kouchner was at the centre of the deepening nuclear standoff between Tehran and the West on Wednesday as Iran's Foreign Ministry summoned a French diplomat to protest at his comments last month that the world should prepare for a war with Iran.

Kouchner said his remarks were taken out of context. On Tuesday he said the situation in Iran was dangerous and that a nuclear-armed Iran would make the Middle East situation even more complicated.

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