US Republican presidential candidate Carly Fiorina has criticized Russia and China over nuclear negotiations with Iran, saying those countries will benefit from a possible agreement with Tehran.

“China and Russia have not been negotiating on our side of the table,” she told ABC News on Sunday.

“It is in those two countries' interests that Iran's economy is open,” she said. “And so in many ways they have been negotiating on Iran's side of the table.”

The former chief executive of Hewlett-Packard software company also attacked American negotiators for caving in to President Barack Obama’s goals.

“I would have walked away because if you can’t walk away from the negotiating table, the other side just keeps negotiating,” Fiorina said.

“We have caved on every major goal that President Obama set,” she claimed.

The Obama administration’s critics have voiced concern as Washington and its negotiating partners are in talks with the Islamic Republic in Vienna to finalize a nuclear deal.

Senate Foreign Relations Committee Chairman Bob Corker said he is “concerned” about the potential deal.

“We have to make sure this is verifiable,” the senator said on Sunday.

Secretary of State John Kerry, who is in Vienna for the talks, said over the weekend that he remained “hopeful” to reach a nuclear deal.

"I think we're getting to some real decisions. So I will say, because we have a few tough things to do, I remain hopeful. Hopeful," he said.

Iran and the five permanent members of the UN Security Council – the United States, Britain, France, Russia and China – plus Germany have been holding intensive talks over the past 16 days to iron out the remaining differences and nail down a landmark deal on Tehran’s nuclear program.