An al-Qaida-linked Filipino terror group has threatened to execute two German hostages it kidnapped in April unless Germany pays a ransom and ceases its support to US-led air strikes against the Islamic State (previously known as Isis) in Syria and Iraq.

Abu Sayyaf, which started kidnapping foreigners in the early 2000s, published a message on Twitter saying it would kill one of the two hostages if its demands were not met within 15 days, according to SITE Intelligence group.

"A message attributed to the Philippines-based Abu Sayyaf Group threatened that two German hostages will be killed unless it is paid a ransom and Germany stops its support to the United States against the Islamic State (IS)," SITE reported.

The German nationals, doctor Stefan Viktor Okonek and his companion Henrite Dielen, were reportedly abducted at gunpoint by the group from a yacht between Malaysia's Sabah state on Borneo Island and the western Philippine province of Palawan in April and then taken by boat to predominantly Muslim Sulu, 950km south of Manila.

The Islamist militants are holding about 10 hostages in the Sulu jungles, including two European birdwatchers, a Japanese and some Filipinos, according to AP reports. The Philippine extremists demand a ransom of 250 million pesos (£3.4 million) for the German captives' release.

In 2001, the Islamist rebels beheaded an American in the southern island province of Basilan who had been seized from an island resort in Palawan province.