The White House hailed the "positive attitude" from Iran and world powers at the negotiating table on Saturday, calling the nuclear talks a "positive first step."

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.