The U.S. Embassy in Qatar warned American citizens on Monday to shelter in place, and a few hours later, Qatar’s foreign ministry announced the country would be closing its air space temporarily. CBS News has learned that the closure of the airspace was in response to a credible threat. The Reuters and AP news agencies are reporting that explosions have been heard across Doha, the capital.

Al-Udeid Air Base in Qatar is the largest U.S. base in the Middle East. It was a major staging ground for U.S. military assets during the wars in Iraq and Afghanistan, and President Trump visited troops at the facility just last month. There are some 8,000 U.S. troops based at al-Udeid.