Washington (AFP) - US President Barack Obama warned Tuesday he will "walk away" from a bad nuclear deal with Iran, as Tehran and six world powers gave themselves until July 7 to reach an agreement.

Insisting there must be a "strong, rigorous verification mechanism" for monitoring Iran's nuclear sites, Obama sent a fresh warning to Iran's leaders and negotiators.

Obama said his instructions to negotiators in Vienna had been "extremely clear" -- that a deal must block Iran from getting a nuclear weapon.

He said there had been "a lot of talk" from Iranian negotiators questioning the terms of a framework agreement reached in Switzerland in April.

"If they cannot, that's going to be a problem because I've said from the start, I will walk away from the negotiations if, in fact it's a bad deal," he told a joint press conference with visiting Brazilian President Dilma Rousseff.

"If we can't provide assurances that the pathways for Iran obtaining a nuclear weapon are closed and if we can't verify that, if the inspections regime, verifications regime, is inadequate, then we're not going to get a deal."

Obama said that "ultimately, this is going to be up to the Iranians."

Iran and the so-called P5+1 -- the United States, Britain, China, France, Russia and Germany -- on Tuesday gave themselves an extra week to clinch a historic nuclear deal as a midnight deadline approached with no breakthrough in sight.

By AFP