Deputy National Security Adviser Ben Rhodes reiterated the White House's call for Tehran to take "concrete steps" to demonstrate that it will not militarize its nuclear program, which the Iranian regime insists serves peaceful, civilian energy purposes.

He made the comments to reporters after Iran and the P5+1 bloc that groups Britain, China, France, Germany, Russia and the United States agreed to more in-depth talks in Baghdad on May 23.

Speaking in Cartagena, on the sidelines of a Summit of the Americas in Colombia, Rhodes welcomed the follow-on meeting as an "additional positive sign."

EU foreign policy chief Catherine Ashton said there had been "constructive and useful" talks with Iranian chief negotiator Saeed Jalili and said this was a "process which, if it is to be successful, will have to be sustained.

But she warned Tehran that "very concrete" results must be achieved at the next meeting.

The UN Security Council has slapped four rounds of sanctions on Tehran over suspicions, driven by the West and Israel, that Iran is spurning the international community with its nuclear program.

The international community's main concern, particularly for Iran's arch foe Israel, is Tehran's growing capacity to enrich uranium, which can be used for peaceful purposes but, when purified further, for a nuclear weapon.

Of special concern has been Iran's formerly secret Fordo site in a mountain bunker near the holy city of Qom, which is enriching to 20 percent purity but, experts say, could be reconfigured to produce 90 percent weapons-grade material.

Fordo's expansion plus a major UN atomic agency report in November on alleged "weaponization" efforts have led to tighter EU and US sanctions on Iran's oil sector, which are due to bite this summer, as well as talk of Israeli military strikes.

"My initial impression is that Iran has been given a 'freebie'," Netanyahu said during talks with visiting US Senator Joe Lieberman, the premier's office reported.

"It has got five weeks to continue enrichment without any limitation, any inhibition. I think Iran should take immediate steps to stop all enrichment, take out all enrichment material and dismantle the nuclear facility in Qom," he said.

"I believe that the world's greatest practitioner of terrorism must not have the opportunity to develop atomic bombs," he said.

At Saturday's talks in Istanbul, during which Iranian officials sat down with the so-called P5+1 grouping of diplomats from the United States, Russia, China, Britain, France and Germany, the sides agreed to hold a more in-depth meeting in Baghdad on May 23.

Officials described the talks as positive but stressed that a great deal was expected of Iran at the next meeting.

The UN Security Council has slapped four rounds of sanctions on Tehran over suspicions harboured by Israel and much of the West that Iran is seeking a militarised nuclear capability -- a charge which Tehran denies.

Of special concern is Iran's formerly secret Fordo site located deep inside a mountain bunker near the holy city of Qom that is enriching to 20 percent purity but which experts say could be reconfigured to produce 90 percent weapons-grade material.

Israel, which sees a nuclear Iran as an existential threat, has not ruled out a preemptive strike on Iranian nuclear facilities.

Iran spurns US request for bilateral nuclear talks: source
Istanbul (AFP) April 14, 2012 - Iran's delegation to crunch talks with world powers on its nuclear programme Saturday turned down a US request for what would have been a rare bilateral meeting on the sidelines, a source told AFP.

"Their request was presented numerous times but Iran has refused," said the source close to the Islamic republic's team at the talks with six world powers in Istanbul, which diplomats said were nonetheless going well.

Earlier diplomats at the talks had said that the US delegation led by Wendy Sherman, undersecretary for political affairs, had let Iran know she was ready for face-to-face discussions with her counterpart Saeed Jalili.

The message was passed to the Iranians by Catherine Ashton, the EU foreign policy chief, on the sidelines of the discussions between Iran and the United States, China, Russia, Britain, France and Germany, envoys said.

The last time that the US and Iran held bilateral talks about the nuclear issue was on the sidelines of a similar meeting in Geneva on October 1, 2009, when William Burns, the senior official leading Washington's delegation, met Jalili.

Despite the refusal, diplomats said that a positive new Iranian attitude in Istanbul raised the prospect for a second round of more in-depth discussions, with one envoy saying this should take place in less than four weeks.

Western powers suspect Iran's nuclear programme is a cover for a secret atomic weapons drive, a charge Tehran vigorously denies. The UN Security Council has passed four rounds of sanctions on the country.

The main concern of the international community, particularly for Iran's arch foe Israel, is Tehran's growing capacity to enrich uranium, which can be used for peaceful uses but when purified further for a nuclear weapon.