The United States and its Arab allies bombed Islamic State targets inside Syria for the first time on Tuesday. The Sunni Muslim group has seized swathes of territory in civil war-torn Syria and Iraq, slaughtering prisoners and ordering Shi'ites and non-Muslims to convert or die.

Germany has ruled out taking part in air strikes, but did break a post-World War Two taboo on sending weapons to active conflict zones by agreeing to arm Kurdish fighters battling Islamic State fighters in northern Iraq.

A Philippine military intelligence source said he was aware of the threats to the German hostages from Abu Sayyaf, but doubted they would be carried out, predicting that the group would most likely negotiate a lower ransom.

"We take all threats seriously," he said. "But, based on our experience in dealing with this group, they are plain criminals who are only interested in getting money. They will eventually settle for a smaller 'board and lodging fee'."

According to media reports, the two Germans, a man and a woman, were seized at gunpoint from a yacht between Malaysian Borneo and the southern Philippines in April.

Abu Sayyaf has been blamed for beheadings, bombings and kidnappings for ransom. The group is also holding a Dutch, a Swiss, a Japanese and some Filipinos in the south of the mainly Roman Catholic state.

In 2001, Abu Sayyaf rebels in the southern island province of Basilan beheaded an American who had been taken captive from an island resort in Palawan province. Two other Americans were held for more than a year, and one was killed during a rescue operation. The other survived with minor wounds.

About 200 U.S. special forces troops have been deployed in the southern Philippines since 2002 to help train and advise local soldiers in fighting Islamist extremists.