Democratic presidential frontrunner Hillary Clinton expressed firm support Wednesday for the nuclear accord with Iran, saying it was flawed but still "strong".

Clinton said the agreement must be strictly enforced and that if elected president next year, she would not hesitate to use military force should Iran fail to live up to its word and try to develop a bomb.

The July deal struck with Iran by the United States and other world powers provides Tehran relief from crippling economic sanctions in exchange for limits on its nuclear program.

Republicans and other critics complain that it does not do away with the program altogether, fails to provide for spot inspections of nuclear sites or force Iran to end support for militant groups like Hamas.

"Is it perfect? Well, of course not," Clinton said in a speech at the Brookings Institution. "No agreement like this ever is. But is it a strong agreement? Yes, it is."

"Either we move forward on the path of diplomacy and seize this chance to block Iran's path to a nuclear weapon, or we turn down a more dangerous path leading to a far less certain and riskier future.

"That's why I support this deal," said the former top diplomat.

As US secretary of state from 2009 to 2013, Clinton played a key role in Obama's policy toward Iran, which blended toughness with negotiation. In particular she was at the forefront of the adoption in 2010 of stinging UN economic sanctions against Iran.

- Military option left open -

Clinton argued that the accord is positive for the security of Israel, which vehemently opposes it.

Republicans in the US Congress and many Democrats are also opposed. But President Barack Obama has won enough support to guarantee it will survive any attempt to shoot it down in an imminent vote.

Largely echoing the position the Obama administration has held for months, Clinton insisted America would never let the Iranians develop a nuclear weapon.

"As president, I will take whatever action is necessary to protect the United States and our allies. I will not hesitate to take military action if Iran attempts to obtain a nuclear weapon," she said.

"There is absolutely no reason to trust Iran."

She added that the United States should be prepared to reimpose sanctions against Tehran, unilaterally if necessary.

Since it was signed on July 14 in Vienna Clinton has expressed her support for the accord but stressed that this was just the beginning of a long process.

Since ending her stint as secretary of state Clinton has at times expressed disagreement with Obama on foreign policy.

But in Wednesday's speech, several times she used the wording "the president and I" -- a way of emphasizing that the Vienna accord was the end product of a policy in which she played a crucial role.

As usual she seemed much more at ease speaking about foreign policy than about the email scandal that is dogging her White House run.

Clinton finally apologized Tuesday night for having a used a private email server rather than the State Department one while serving as secretary of state. Her practice is being investigated by US authorities because some of the emails have been retroactively rated as classified.

In her speech Clinton also suggested the United Nations hold an urgent meeting on how to cope with the flood of Syrian refugees fleeing to Europe to escape the war ravaging their country.