Tehran/Vienna: Iran’s President Hassan Rouhani said Monday that a nuclear deal with world powers would be done despite a missed deadline in Vienna that prompted a seven-month extension in talks.

“This way of negotiation will reach a final agreement. Most of the gaps have been removed,” he said on state television, referring to the main differences that have so far prevented an interim deal being turned into a comprehensive settlement.

Iran and world powers failed in an enormous diplomatic push to seal a landmark nuclear deal by a Monday deadline, deciding instead to give themselves seven more months to reach agreement.

In their second extension this year, Iran and the five permanent members of the UN Security Council plus Germany will seek to strike an outline deal by March and to nail down a full technical accord by July 1, officials said.

US Secretary of State John Kerry defended extending a deadline for a nuclear deal with Iran, saying “real and substantial progress” was made in talks in Vienna and calling on US lawmakers not to impose new sanctions on Tehran.

“We have made real and substantial progress,” Kerry said in Vienna following nearly a week of talks involving six world powers and Iran.

However Three influential US Republican senators said on Monday the extension of nuclear negotiations with Iran should be coupled with increased sanctions and a requirement that any final agreement be sent to Congress for approval.

The three senators - John McCain, Lindsey Graham and Kelly Ayotte - said in a statement that they view Iran’s insistence on having any enrichment program as problematic, and warned that a “bad deal” with Iran would start a nuclear arms race in the Middle East.

“This is certainly not the time to get up and walk away... We look for your support (in Congress) for this extension,” said Kerry.

Republicans warn President Barack Obama is being fooled by the new, more moderate face of the Islamic republic, which it claims aims to win billions of dollars in sanctions relief and will still covertly seek to develop a nuclear weapon.

Legislation is already pending before US lawmakers that instead of lifting sanctions would impose even harsher ones.

But Kerry added: “We would be fools to walk away from a situation where the breakout time has already been expanded rather than narrowed and where the world is safer because this program is in place.”

The breakout time is generally taken to mean how long Iran would need, were it to choose to do so - and it hotly denies such an aim - to produce one bomb’s worth of fissile material.

Kerry conceded the negotiations over the next few months would not be any easier just because there is an extension.

“They’re tough and they’re going to stay tough,” he told hundreds of reporters crowded into a tent outside the 19th century palace where the talks took place.

But he said “today we are closer to a deal that would make the entire world, especially our allies and partners in Israel and in the (Arab) gulf, safer and more secure.”