Secretary of State Rex Tillerson must declare Iran non-compliant with the nuclear agreement in order to avoid "rewarding Iran's belligerence," according to a group of Republican senators.

"We believe that a change in that policy is long overdue," Arkansas Republican Sen. Tom Cotton and three colleagues wrote to Tillerson in a letter Tuesday.

"In light of Iran's malign actions since the signing of the [nuclear deal], the only reasonable conclusion is that the full suspension of U.S. sanctions is not in the vital national security interests of the United States and that Iran has consistently violated the terms of the [nuclear deal]."

If Tillerson were to follow their advice, that would represent a significant step towards scrapping the international agreement that former President Barack Obama and his administration negotiated with Iran and other western powers.

Under federal law, a finding that Iran is not complying with the deal — the certification must take place every 90 days — would set the stage for "an expedited process for Congress to rapidly restore its sanctions." Cotton and the other senators said that time has come.

"We believe that a change in that policy is long overdue," they wrote. "What would be highly imprudent is to continue the Obama-era practice of offering sheepish and faint-hearted certifications as a matter of course, hoping no one takes notice. That is the surest way to encourage Iran's campaign of imperial aggression and speed its progress toward nuclear breakout."

Tillerson certified Iranian compliance with the deal when the question arose for the first time in the Trump administration, but issued accompanying remarks that suggested he was maintaining the Obama-era status quo only temporarily.

"Strategic patience is a failed approach," Tillerson said. "The Trump administration has no intention of passing the buck to a future administration on Iran."

The senators' letter suggests some Republicans, even White House partners, are disinclined to exercise similar patience with the administration's review of its policy on Iran.

The letter was signed by Republican Georgia Sen. David Perdue, one of Trump's top congressional allies, as well as erstwhile rivals Ted Cruz of Texas and Marco Rubio of Florida. They chronicled a handful of violations of the agreement, such as "Iran's refusal to grant international inspectors access to nuclear-research and military facilities," but argued as well that Iranian aggression unrelated to the deal should trigger the policy change.

"Even if we put aside issues related to Iran's violations of the [nuclear deal], the full suspension of U.S. sanctions falls far short of being 'vital to the national security interests of the United States,' as required by [federal law]," they wrote. "In fact, a continuation of current policy would be tantamount to rewarding Iran's belligerence."