Air Canada and Singapore Airlines have joined Korean Air and Asiana in diverting flights away from North Korean airspace after the communist government said it could not guarantee the safety of passenger jets flying near its shores or southern border. The threat comes as the U.S. and South Korea prepare to begin on Monday annual military exercises North Korea considers provocative.

The change in routing affects a single westbound Air Canada flight between Vancouver and Seoul, an Air Canada spokesman told AIN. It does not affect the eastbound flight, he said. Although a Singapore Airlines spokesperson also confirmed that it has rerouted some flights as well, she could not offer details about which routes stand affected. “We do these things on a day to day basis,” she said. “We observe the political developments and if anything major happens we react appropriately.”

Korean Air and Asiana began rerouting flights to and from North America immediately after North Korea issued its threat yesterday. Asiana, for one, has reportedly said it would detour through Japanese airspace, adding about 40 minutes to each leg. Plans call for the U.S.-South Korean military exercises to last 12 days.