President Donald Trump said that Iran had not lived up to the spirit of the nuclear deal agreed with world powers and suggested he would reveal his decision on whether to certify the agreement soon.

"We must not allow Iran ... to obtain nuclear weapons," Trump said during a meeting with military leaders at the White House on Thursday.

"The Iranian regime supports terrorism and exports violence, bloodshed and chaos across the Middle East. That is why we must put an end to Iran's continued aggression and nuclear ambitions. They have not lived up to the spirit of their agreement," he said.

Asked about his decision on whether to certify or decertify the landmark nuclear deal, Trump said: "You'll be hearing about Iran very shortly."

READ MORE: Donald Trump denounces Iran over nuclear deal

Trump is expected to announce his decision October 12, the Washington Post reported, but wrote he would likely avoid recommending the US reimpose sanctions on Iran.

'Worst deal ever negotiated'

If Trump, who has called the accord "the worst deal ever negotiated", does not recertify it by October 16, Congress has 60 days to decide whether to reimpose sanctions suspended under the accord.

The agreement provided Iran with billions of dollars in sanctions relief in exchange for widespread curbs on and access to its nuclear programme.

James Mattis, the defence secretary, told legislators on Tuesday that it is in the best interest of US national security to remain in the deal.

"If we can confirm that Iran is living by the agreement, if we can determine that this is in our best interests, then clearly, we should stay with it," Mattis testified before the Senate Armed Services Committee.

"I believe at this point in time, absent indications to the contrary, it is something the president should consider staying with."

During the same hearing, General Joseph Dunford said he believed Iran is not in material breach of the accord, which he said has delayed Tehran's nuclear capability.

READ MORE: UN nuclear watchdog defends Iran agreement

The White House said Trump's team has "presented a united strategy that the national security team all stands behind and supports. And the president will make that announcement soon."

A unilateral US exit from the agreement would likely have undesirable consequences for Washington - isolating it from its negotiating partners that include close European allies, and potentially forcing Washington to sanction them if they continue to keep with the accord's parameters.

Iran's options

In September, Iranian Foreign Minister Mohammad Javad Zarif told Al Jazeera his country may also abandon the deal if the US decides to withdraw from it.

The deal is supported by the other major powers that negotiated it with Iran and its collapse could trigger a regional arms race and worsen tensions in the Middle East.

"If Washington decides to pull out of the deal, Iran has the option of withdrawal and other options," Zarif said, adding that, "Washington will be in a better position if it remains committed to the deal."

Iranian authorities have repeatedly said Tehran would not be the first to violate the agreement, under which Tehran agreed to restrict its nuclear programme in return for lifting most international sanctions that had crippled its economy.

The prospect that Washington could renege on the deal has worried some of the US allies that helped negotiate it. French President Emmanuel Macron said last week that there was no alternative to the nuclear accord.