Leading Republican presidential candidate Donald Trump has broken away from the GOP pack on the Iran nuclear agreement, saying if he becomes the US president, he will strictly enforce the Vienna accord.

A day after signing the Republican Party’s loyalty pledge and ruling out the prospect of a third-party White House run, Trump said on Friday he would comply with the Obama administration's nuclear deal with Iran.

The maverick New York billionaire, nonetheless, called the historic accord "a disastrous deal" and "a horrible contract."

"I love to buy bad contracts where key people go bust, and I make those contracts good," he said.

Trump’s rivals seeking the Republican Party’s nomination for the 2016 presidential election have opposed the nuclear pact and many have even vowed to undo it.

Iran and the P5+1 group of countries – the US, Britain, France, Russia, China, and Germany – announced the conclusion of nuclear negotiations in the Austrian capital, Vienna, on July 14.

Under the agreement, Iran has been recognized by the United Nations as a nuclear power and will continue its uranium enrichment program, but some restrictions will be placed on Iran’s nuclear program in exchange for the removal of sanctions.

Republicans, who control both houses of Congress, oppose the agreement because they say it gives too many concessions to Iran and threatens the security of Israel.

Congress is reviewing the nuclear agreement and has until September 17 to vote to either approve or disapprove of it.

Democrats need at least 34 votes in the Senate, or 146 in the House of Representatives, to sustain the veto President Barack Obama has promised.

So far 38 senators have announced to vote in favor of the nuclear agreement in Congress, meaning Democrats will now have enough votes to protect it.