The United States issued a new warning to airlines about operating jets over Iran, urging them to exercise caution and citing concerns over military activity including an unnamed US civil operator being intercepted by fighter jets in December 2017. The updated guidance from the Federal Aviation Administration to US 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. Flight Service Bureau, which provides safety information on airspace 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 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 US based group said in an email to clients on Monday. The US 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. 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 Tehran last month.