US commandos have mounted a rare raid into eastern Syria overnight, killing a senior Islamic State commander in a firefight, capturing his wife and rescuing a Yazidi woman held as a slave, the Pentagon said.

Islamic State militants are said to be advancing toward the historic city of Palmyra in central Syria, seizing an oil field and taking control of the water company on the outskirts.

Mortars rained down on a crowded marketplace in a Palestinian refugee camp in the Syrian capital, killing 21 people as regime forces and rebels clashed on the southern outskirts of Damascus, activists have said.