Syria warned its Palestinian refugee population on Monday not to aid the insurgency that is fighting President Bashar al-Assad, as hundreds of Palestinians fled the Yarmouk neighborhood of Damascus. Many headed for relative safety in Lebanon, a day after Syrian forces attacked that neighborhood with airstrikes for the first time in the civil war.

The Syrian warning appeared to reflect the importance that Mr. Assad attaches to the loyalty of the country’s Palestinians, an important element of what remains of his political legitimacy. It came as new clashes were reported in and around the Yarmouk neighborhood between government forces and rebel fighters.