Press undersecretary Roberto Capco said on Tuesday the videotape was seized in 1996 at an Abu Sayyaf camp on Basilan island.
US special forces have been deployed at Basilan to observe Filipino troops hunting the guerrillas, who have been holding an American missionary couple and a Filipino nurse hostage for nearly nine months.
"It's to show to the public the real face of the enemy," Capco said.
The poor-quality tape - a compilation of several clips made over the previous two years - was aired late on Monday by at least two Philippine television networks. One showed the graphic footage complete; the other pixilized the beheadings.