Washington, Vienna

US President Barack Obama on Tuesday warned 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 said.

“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.”

In Tehran, President Hassan Rouhani warned that Iran would resume its halted nuclear work if they went back on a proposed final deal aimed at curbing the country’s nuclear work in return for easing sanctions.

“If we reach a deal, both sides should be committed to it,” Rouhani said in Tehran, IRNA reported. “If the other side breaches the deal, we will go back to the old path, stronger than what they can imagine.”

In Vienna, Russian Foreign Minister Sergei Lavrov said however he believes that, after almost two years of trying, a deal ending the 13-year standoff is “within reach”.

The talks are “progressing in a positive direction. There remain questions, mostly regarding procedural issues rather than technical,” he said after meeting US Secretary of State John Kerry.

Earlier in the day, Iranian Foreign Minister Mohammad Javad Zarif returned to Vienna following consultations in Tehran, a visit that had raised hopes he may be bringing instructions that would yield a breakthrough.

But after Kerry met with Zarif for almost two hours, the US State Department said the P5+1 group of global powers had agreed to extend the terms of an interim agreement until July 7.

A State Department official said however that this did “not necessarily mean they will go until the 7th or end on the 7th.”

Kerry had also made no plans for when he might leave the Austrian capital.

Supreme leader Ayatollah Ali Khamenei appeared to give Zarif his blessing, tweeting in English Tuesday: “I recognise our negotiators as trustworthy, committed, brave and faithful.”