The victims include 13 rebels and eight soldiers.
Areas controlled by the Moro Islamic Liberation Front (MILF) near the town of Mamasapano on Mindanao island were pounded by helicopter gunships after the rebels attacked an army outpost in violation of a two-year-old ceasefire, said Aljazeera.
Military spokesman Colonel Franklin del Prado of the Philippine army's 6th Infantry Division in the area said the slain soldiers all belonged to the battalion base attacked by the MILF.
Reports from the field indicated that at least 13 MILF rebels died in the counter-offensive, he added.