ZAMBOANGA, Philippines ? The father of a kidnapped American missionary Thursday accused the Muslim extremist Abu Sayyaf group of reneging on an agreement to free his son and daughter-in-law.

Paul Burnham called Radio Mindanao Network in the southern Philippines from his home in Wichita, Kan., saying the Abu Sayyaf had added extra, unspecified conditions to the deal he said was finalized March 13 and also was to include the release of Filipino nurse Ediborah Yap.

Missionaries Martin and Gracia Burnham of Wichita and Yap are the last hostages remaining from an Abu Sayyaf kidnapping spree that began a year ago. Some captives have escaped or have been released, reportedly for ransom, and others have been beheaded, including American Guillermo Sobero.

“The family of Martin and Gracia are deeply saddened and disappointed to learn that the Abu Sayyaf has broadcast its intention to continue holding Martin and Gracia and Deborah Yap until additional demands are met,” Paul Burnham said in a statement carried by the radio station.

It was unclear where Abu Sayyaf had broadcast that intention.

Burnham said a deal had been reached through a mediator, Abu Sulaiman, on March 13. He said Sulaiman told the family on March 19 that the Abu Sayyaf had agreed to a five-day window on freeing the hostages.

“On March 26, Abu Sulaiman advised us to tell Martin and Gracia’s children that they would be released soon,” Paul Burnham said. “The children and the family have been anxiously waiting for them to abide by their word.

“But it now seems that they do not intend to keep their promises to us, and can any family believe the promises of the Abu Sayyaf in the future? Is it futile to make agreements with them?”

Senior U.S. officials said on April 12 they were monitoring negotiations for the Burnhams’ release that were being conducted by a group known to the rebels and that the talks were at a delicate stage.

The Abu Sayyaf has been dragging the hostages through the jungles of Basilan Island, 550 miles south of Manila, pursued by thousands of Philippine troops.

A man claiming to speak for the Abu Sayyaf claimed responsibility for bombings Sunday in nearby General Santos city that killed 15 people and injured 71.