Muslim rebels in the Philippines have threatened to kill 20 hostages, including an American missionary couple, abducted from a beach resort Sunday morning.

New Tribes Mission (NTM) reports that Martin and Gracia Burnham, missionaries with the organization for 16 years, were staying at the resort to celebrate their 18th wedding anniversary.

The Abu Sayyaf rebels have taken responsibility for the kidnappings, and allowed Martin Burnham to speak over the radio Monday. "I along with my wife Gracia are in the custody of the group," the missionary pilot said. "We are safe, we are unharmed, our needs are being met and we would like to appeal to all for reasonable and safe negotiations."

President Gloria Macapagal Arroyo has vowed to crush the abductors, telling them, "It's better for you to free the hostages you abducted while there is still time. Otherwise, bullets will rain on you." But NTM spokesman Scott Ross tells The Orlando Sentinel that such words could mean the death of the Burnhams.

"The Philippines government appears to be taking a very strong line, and that could be adverse to the safety of our people," Ross told the paper. "The rhetoric we're hearing from the Philippine government is very strong."

Indeed, the rebels say they will kill the hostages at the first sign of military force. "We will not think twice to have a mass killing of the hostages once the military will launch an all-out offensive," a spokesman for the group said.

The Burnhams have three children, who did not join the couple on their anniversary trip.

The raid is not the first kidnapping of NTM missionaries. In a highly publicized 1993 case, NTM's Dave Mankins, Rick Tenenoff, and Mark Rich were abducted by from a Panamanian village near the Colombian border by Marxist rebels. Their whereabouts and welfare remain unknown.

Kidnapping has become increasingly common, especially around the Philippines, where the Abu Sayyaf and others have seized dozens of people for political and financial reasons.