Iran temporarily closed its airspace early on Thursday morning, issuing a NOTAM (Notice to Airmen) to all flights except international ones to and from Iran with official permission, as Western military officials say US military action in the country is "imminent."

Flight tracker websites showed the airspace over Iran and Iraq emptying rapidly prior to the NOTAM's issuing. The prohibition lasted more than two hours and ended at around 2:30 a.m. Israel time.

India's largest airline, IndiGo, said some of its international flights were impacted by Iran's sudden airspace closure. A flight by Russia's Aeroflot bound for Tehran returned to Moscow after the closure, according to tracking data from Flightradar24.

"All the signals are that a US attack is imminent, but that is also how this administration behaves to keep everyone on their toes. Unpredictability is part of the strategy," a Western military official told Reuters later on Wednesday.

Shortly before the NOTAM was issued, US President Donald Trump held a press conference in the Oval Office, in which he said he was told that the killings in Iran were "stopping."

When asked if the US was still considering military action, Trump answered, "We're going to watch and see what the process is."

Both Poland and Italy have urged their citizens to leave Iran immediately. The British embassy in Tehran has been temporarily closed. Less than 24 hours before, the US had urged its own citizens to leave. 

The US has also withdrawn military personnel from the US military's Al Udeid Air Base in Qatar.

Earlier on Wednesday, a senior Iranian official told Reuters that Iran warned regional countries that it would strike US military bases in those countries in case of a US attack.

This is a developing story.