US President Barack Obama said Sunday that nothing had been "given away" to Iran at weekend nuclear talks in Istanbul, warning Tehran that the clock was still ticking to reach an accord.

Obama's comments came after Israeli Prime Minister Benjamin Netanyahu said his "initial impression is that Iran has been given a 'freebie'" at a first round of key talks with world powers over its disputed nuclear program.

"The clock is ticking and I've been very clear to Iran and our negotiating partners that we're not going to have these talks just drag out in a stalling process," Obama told reporters after an Americas summit in Colombia.

"But so far at least we haven't given away anything -- other than the opportunity for us to negotiate," 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 the Islamic republic at the next meeting.

Netanyahu -- whose government has not ruled out a preemptive strike on Iranian nuclear facilities -- earlier said however that Tehran had simply bought itself some extra time to comply.

"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.

But Obama refuted that statement, saying "The notion that we've given something away or a freebie would indicate that Iran has gotten something."

"In fact, they got the toughest sanctions that they're going to be facing coming up in a few months if they don't take advantage of those talks. I hope they do," Obama said.

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