Iran restricted its airspace to commercial planes for more than four hours early Thursday, providing no explanation as tensions with the United States remained high over Tehran's suppression of demonstrations.
The closure came as US President Donald Trump said Wednesday he has not ruled out military action against Iran.
According to pilot guidance issued by Iran, which lies on a key East-West flight route, the closure saw international carriers diverted north and south around the country.
"Several airlines have already reduced or suspended services, and most carriers are avoiding Iranian airspace," said SafeAirspace, a website that provides information on conflict areas and air travel.
"The situation may signal further security or military activity, including the risk of missile launches or heightened air defence, increasing the risk of misidentification of civil traffic."
Media reports suggest the closure, which rippled through global aviation, appeared to have expired after one extension. Several domestic flights were in the air just after 7 am local time.
The airspace closure came as some personnel at a key US military base in Qatar were advised to evacuate. The US embassy in Kuwait ordered its personnel to "temporarily halt" going to multiple military bases in the Gulf nation.
Iran previously shut its airspace during the 12-day conflict with Israel in June, when it exchanged fire with Israel during the Israel-Hamas war.
Trump claims killing of Iran protesters ‘has stopped’
Trump said Wednesday he had been told plans for executions in Iran have stopped, without providing details. The shift came a day after Trump told protesters in Iran that "help is on the way" and his administration would "act accordingly" to respond to the Islamic Republic's deadly crackdown.
Iranian Foreign Minister Abbas Araghchi sought to tone down the rhetoric, urging the US to find a solution through negotiation.
Asked by Fox News what he would say to Trump, Araghchi said: "My message is: Between war and diplomacy, diplomacy is a better way, although we don't have any positive experience from the United States. But still diplomacy is much better than war."
The change in tone between Washington and Tehran came hours after the chief of the Iranian judiciary said the government must act quickly to punish thousands who have been detained.
Activists warned hangings of detainees could come soon. Trump made a vague statement Wednesday that he's been told "on good authority" that plans for executions stopped, even as Tehran has indicated fast trials and executions ahead in its crackdown on protesters.
"We've been told that the killing in Iran is stopping — it's stopped — it's stopping," Trump said at the White House while signing executive orders and legislation. "And there's no plan for executions, or an execution, or executions — so I've been told that on good authority."
Nearly 3,500 people have been killed in Iran's security forces' crackdown on demonstrations, according to the US-based Human Rights Activists News Agency.
The death toll exceeds that of any other round of protest or unrest in Iran in decades and recalls the chaos surrounding the country's 1979 Islamic Revolution.
Insiders within Iran have told Euronews that the death toll is feared to be considerably higher, with as many as 15,000 Iranians killed as the Tehran regime reportedly further increased its violent crackdown against the demonstrations in recent days.
Trump, who has vowed to act militarily on the issue, on Monday imposed 25 per cent tariffs on countries doing business with Tehran and urged Iranians to keep protesting, saying "help is on the way" and his administration would "act accordingly."
The US president has not offered details about how the US might respond. It remains unclear whether he would follow through on threats of military action and what that might entail.
