The United States has issued a fresh warning to airlines to exercise caution when operating in Iran's airspace, citing concerns over military activity including an unnamed U.S. civil operator being intercepted by fighter jets in December 2017.

The updated guidance from the Federal Aviation Administration to U.S. operators, issued on Sunday at the expiry of the prior year's advisory, said there were also military activities emanating from or transiting through Iran's airspace associated with the conflict in Syria.

Tensions ramped up between Iran and the United States after President Donald Trump pulled out of a landmark nuclear deal with Iran in May and reimposed sanctions on the Islamic Republic last month.

Flight Service Bureau, which provides safety information on airspace to airlines, said "without seeming alarmist", the deteriorating relationship between the U.S. and Iran must be taken into account when planning flights in Iran's airspace.

"Although the reopening 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 U.S. based group said in an email to clients on Monday.

The U.S. Department of State advises that its citizens do not travel to Iran due to the risk of arbitrary arrest and detention. Flight Service Bureau said that could present problems in the event of an unplanned landing in Iran for medical or technical reasons.

For Iraq, the U.S. Department of State advises its citizens against travel to the country due to terrorism and armed conflict. The F.A.A.'s latest guidance on Iraq, issued in December 2017, prohibits U.S. airlines in most cases from flying at an altitude lower than 26,000 feet due to the potential for fighting.