Republican rivals for the US presidential candidacy Donald Trump and Ted Cruz joined forces on Wednesday to denounce the international deal with Iran over its nuclear program, which is currently being debated in Congress.

Thousands of people gathered on the west lawn of the Capitol to hear the two speak out against the agreement and to protest President Obama's role in securing it.

Trump blasted the "incompetently negotiated" deal and rebuked the president for not making the release of four American citizens held in Iranian prisons one of the terms.

"If I win the presidency, I guarantee you that those four prisoners are back in our country before I ever take office," Trump shouted to a jubilant crowd.

"We are a country that owes $19 trillion," he added, changing tack to the economy, "we lose everywhere."

"We will have so much winning if I get elected that you may get bored with winning," Trump said, making repeated use of his favorite campaign trail buzzwords.

Cruz: deal is supporting anti-American jihadists

Keen to bask in Trump's limelight, it was Cruz who invited the real estate mogul to the event. The Texas senator also took the stage to raucous applause before accusing Democrats who support the deal of sending "billions of dollars to jihadists who have pledged to murder Americans."

The hard-won deal was finalized in July after more than a decade of negotiations between Iran and several world powers. It will lift crippling economic sanctions for Tehran in exchange for stringent limits on its nuclear program.

Republicans have argued that the agreement is not strong enough, and should indeed do away with Iran's nuclear program altogether.

es/bw (AP, AFP, Reuters)