The United States warned of "disturbing trends" in Iran's behavior Tuesday after Tehran's military threatened to down two US Navy planes over the Strait of Hormuz.

Some in Washington had hoped Iran's regime would show a warmer face to the world after this year's nuclear deal, but a US official admitted this has not been the case.

Asked about Iran's military provocations, its ballistic missile tests and its detention of US citizens, State Department spokesman Mark Toner acknowledged concern.

"We have seen some disturbing, as you cited, trends," he said, adding of the threat to the planes, "we have seen a succession of these events over the past month or so."

The spokesman insisted that Iran would have been an even greater threat to its region and to the United States if it had not signed the deal to limit its nuclear ambitions.

But he admitted it was possible that Iran had stepped up its provocations in other domains after the controversial agreement won the Islamic republic sanctions relief.

"I can't rule that out," Toner said, when asked if the nuclear deal, rather than taming Iran, had emboldened it to adopt a more aggressive stance.

"Iran, like many countries, has an internal political process that is defined by a lot of different dynamics, so I can't give an assessment on that one way or another."

Earlier, the Pentagon had confirmed the latest in a series of dangerous military encounters between the US Navy and Iranian forces near key oil trade routes in the Gulf.

Two US maritime patrol aircraft were flying separate missions in international airspace on September 10 when Iranian air defense batteries challenged them by radio.

US Naval Forces Central Command spokesman Commander Bill Urban said the Iranians told the pilots they "were flying near Iranian airspace, and if they did not change course immediately that 'we will fire Iranian missile'."

"The US aircraft independently replied, 'This is a coalition aircraft conducting routine operations in international airspace,' and continued their respective missions," he added.

The spokesman said the Iranian warning was "unprofessional" but the US Navy has not deemed the incident "unsafe" because the US planes were outside known Iranian anti-aircraft missile ranges.

- Series of blunders -

In recent weeks, the Pentagon has denounced a series of "unsafe and unprofessional" maritime encounters in the Gulf, including one that prompted an American ship to fire warning shots at an Iranian vessel that got too close.

Navy officials say ships from the US and Iranian navies interacted more than 300 times in 2015 and more than 250 times the first half of this year.

Ten percent of these encounters have been deemed unsafe and unprofessional.

In January, the Iranian navy briefly captured the crews of two US patrol boats that had, through a series of blunders, strayed into Iranian territorial waters.

The 10 American sailors were released within 24 hours.