The airport in Kabul is under the threat of an “imminent attack,” possibly within “hours” on Thursday, according to a British defense official.

The threat alert follows the State Department’s warning to U.S. citizens at airport gates on Wednesday to leave “immediately” due to threats. The Hamid Karzai International Airport has been temporarily shuttered numerous times in recent days as U.S. forces aim to complete evacuations and withdrawal from the region by Aug. 31.