The Iranian military threatened to shoot down two US Navy planes flying over the Strait of Hormuz, a defence official told AFP on Tuesday, the latest in a string of encounters with Tehran.

Two “maritime patrol aircraft” were flying separate missions in a similar area in international air space earlier this month when they received three radio calls from Iranian air defence.

“They were threatening to shoot at us, to shoot us down, or fire missiles at us,” the defence official said, speaking on condition of anonymity ahead of a formal announcement.

According to Fox News, which first reported the encounter, the US planes ignored the warning and continued on their mission.

One unnamed defence officials told the news network that the US military had wanted to test the Iranians’ reactions.

The defence official AFP spoke to said the incident was “unprofessional” but was not deemed unsafe because the US planes were outside the bounds of known Iranian anti-aircraft missile ranges.

The Pentagon has in recent weeks 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, with 10% of those encounters 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.