The pilots’ union for American Airlines has directed its members to refuse to fly to Israel, citing the ongoing war between Israel and the Palestinians in Gaza.

Union President Ed Sicher said in an email to members that the company’s pilots should not fly to Israel until they “can be reasonably assured of the region’s safety and security.”

The email cites the most recent advisory from the U.S. State Department, which warns that the current situation in Israel “continues to be unpredictable,” and that mortar and rocket fire can take place any time without warning, putting aircraft in danger.

“It is not prudent or appropriate to knowingly put our flight crews and passengers in harm’s way by maintaining flights into a war zone,” Sicher said.

Israel formally declared war Sunday as it bombarded the Gaza strip with airstrikes in retaliation for a major surprise attack by Hamas.

The declaration came a day after an unprecedented incursion by Hamas fighters, who blew through a fortified border fence and gunned down civilians and soldiers in Israeli communities along the Gaza frontier during a major Jewish holiday.