BEIRUT, Lebanon — 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.

Hundreds of thousands of Palestinians live in Syria, displaced by the Arab-Israeli struggle. Historically, they have considered Mr. Assad a benefactor and an ally. Yarmouk was originally a refugee camp and has developed into a mixed Damascus neighborhood where many Palestinians live. But increasing numbers of them have been siding with the insurgents.

The warning aimed at these Palestinians was conveyed in a news dispatch by SANA, the official news agency, about a telephone conversation between the country’s foreign minister, Walid al-Moallem, and the United Nations secretary general, Ban Ki-moon, concerning the general situation in Syria and specifically the Yarmouk neighborhood.