After weeks of bickering, the Republican presidential candidates showed a united front on Wednesday as they slammed US President Barrack Obama for agreeing to a six-nation nuclear deal with Iran.

Just hours after the deal was reached, Republican Party leaders wasted no time in vowing to reject the agreement when it arrives in Congress for review.

Congress will have 60 days to review the nuclear deal as soon as all documents are sent to Capitol Hills before it can pass a resolution of approval or disapproval or do nothing.

The deal is seen as the crowning glory of President Obama's foreign policy. However, critics see it as an American surrender to Iran, a nation that still regards the US as "the Great Satan."

Former Florida Gov. Jeb Bush, who is leading national polls for the Republican presidential race by a small margin, denounced the deal, calling it "dangerous, deeply flawed and shortsighted."

"The people of Iran, the region, Israel, America and the world deserve better than a deal that consolidates the grip on power of the violent revolutionary clerics who rule Tehran with an iron fist," Bush said.

Bush issued the statement just a few hours after President Barack Obama announced that some economic and financial sanctions on Tehran would gradually be lifted over a period of six months in exchange for Iran's agreement to curb its nuclear arsenal.

Real estate mogul Donald Trump also criticised the controversial nuclear accord with Iran, which he thinks would set a very dangerous precedent that could be catastrophic for Israel.

"The Obama administration's agreement with Iran is very dangerous. Iran developing a nuclear weapon, either through uranium or nuclear fuel, and defying the world is still a very real possibility," Trump said in a statement.

"Every promise the Obama administration made in the beginning of negotiations, including the vow — made at the beginning of the negotiations — to get our great American prisoners returned to the US has been broken," he added.

There are at least four Americans still being held in Iranian prison including Jason Rezaian, who was captured in July 2014; Saeed Abedini, in July 2012; Amir Hekmati, in August 2011; and Robert Levinson, held since March 2007.

Florida Senator Marco Rubio, another Republican presidential hopeful who sits on the Senate Foreign Relations Committee, said the next elected US President needs to renew a tough American stance on Iran's nuclear program.

"Failure by the President to obtain congressional support will tell the Iranians and the world that this is Barack Obama's deal, not an agreement with lasting support from the United States," Rubio said.

"It will then be left to the next President to return us to a position of American strength and re-impose sanctions on this despicable regime until it is truly willing to abandon its nuclear ambitions and is no longer a threat to international security," he added.

Wisconsin Gov. Scott Walker, for his part, called on congressional leaders and his election rivals, including Democratic presidential hopeful and former US Secretary of State Hillary Clinton, to repudiate the agreement.

"Iran's Supreme Leader should know that a future American president will not be bound by this diplomatic retreat. Undoing the damage caused by this deal won't be easy. But when the United States leads, and has a president who isn't eager to embrace Iran, the world will follow," Walker said.

"In order to ensure the safety of America and our allies, the next president must restore bipartisan and international opposition to Iran's nuclear program while standing with our allies to roll back Iran's destructive influence across the Middle East," he added.

Texas Senator Ted Cruz refuses to say what he thinks the next president should do but urged the public to campaign against the deal during the congressional review period.

"Thankfully, it is not a done deal. We still have an opportunity to tell the truth about what [Israeli] Prime Minister Netanyahu called today a 'bad mistake of historic proportion," Cruz said.