The United States has warned airlines about operating in Iran's airspace, claiming military activity there could put civilian aircraft at risk.
Sunday's guidance from the Federal Aviation Administration to American companies referred to an unidentified US civil operator which was intercepted by fighter jets in December last year.
The warning also said there was military activity in Iran's airspace associated with the Syria conflict.
Tensions between Iran and the US increased after President Donald Trump pulled out of a landmark nuclear deal with Tehran in May and reimposed sanctions on the country last month.
Flight Service Bureau, which provides airspace safety information to airlines, said "without seeming alarmist", the deteriorating relationship between the US and Iran must be taken into account when planning flights in Iran's airspace.
"Although the re-opening of Iraqi airspace in November last year has provided additional routing options ... there is no perfect route in the region, and operators must consider their preference for Iraq vs Iran," the US-based group said.
The US Department of State advises its citizens against travel to Iran because of the risk of arbitrary arrest. Flight Service Bureau said this risk could present problems if there was an unplanned landing in Iran for medical or technical reasons.
For Iraq, the FAA's latest guidance from December last year prohibits US airlines in most cases from flying lower than 26,000 feet.