As negotiators from Iran and six world powers prepared for nuclear talks in Switzerland on Wednesday, Supreme Leader Ayatollah Ali Khamenei made Iran’s position clear: It would “not step back one iota” from its nuclear rights.

In a televised address to tens of thousands of volunteer Basij militiamen in the capital Tehran, Iran’s most powerful man said he had set “limits” for the country’s representatives, which “have to be observed.”

Representatives of the P5+1 — the US, UK, France, China and Russia, plus Germany — will hold two days of meetings with Iranian negotiators in Geneva.

Iran's Foreign Minister Mohammed Javad Zarif said Wednesday that he would hold "serious and detailed" discussions with EU foreign policy chief Catherine Ashton on Thursday.

"So far, we have progressed based on our plans and we agreed to start serious and detailed negotiations about the agreement with Lady Ashton tomorrow," Zarif wrote after lunching with Ashton.

Ashton's spokesman said the hope was that differences would be "narrowed this time" after the previous round of talks, when "a great deal of progress" was made.

Abbas Araqchi, Iran's negotiator, told Fars news agency that Wednesday's meetings would focus on the process of negotiations and discussions of any agreement would likely only start Thursday.

It is the second round of talks in the Swiss city this month. The first round failed to reach an agreement, with diplomats blaming Iran’s insistence on its right to enrich uranium and French worries over an Iranian heavy water reactor being built in Arak.

Khamenei’s comments suggest Iran’s position has not softened.

"We do insist that we will not step back one iota from our rights," he said in the speech broadcast live on Iran’s Press TV with English translation, possibly to ensure his message was heard loud and clear by the P5+1 negotiators in Switzerland.

"We do not intervene in the details of these talks. There are certain red lines and limits. These have to be observed."

He added: "They should know that the Iranian nation respects all nations of the world, but we will slap aggressors in the face in such a way they will never forget it."

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Khamenei’s comments came as US President Barack Obama called on senators to hold off on imposing more sanctions against Iran.

During a two-hour closed-door meeting at the White House with Democratic and Republican lawmakers, Obama said they should give diplomacy a chance.

"The president made clear that achieving a peaceful resolution that prevents Iran from obtaining a nuclear weapon is profoundly in America's national security interest," White House spokesman Jay Carney told reporters.

Sanctions have proved crippling to Iran's economy, with oil revenues plummeting and local currency's value dropping 80 percent by 2012.

Meanwhile, Israeli Prime Minister Benjamin Netanyahu was on his way to Russia Wednesday to make the case for a tougher deal. The Israeli leader already dismissed the proposals from earlier talks in Geneva as an "exceedingly bad deal," saying Tehran would still be able to build a bomb with those terms.

Deputy Israeli Foreign Minister Ze'ev Elkin, who was in Moscow with Netanyahu, said, "Our job is to try to sway the Russians, as we have been doing with all the players."