The Islamists have extended the deadline multiple times - first by a week, then by a few hours, if they could ensure the ransom was on its way.

Hours before the original deadline, hundreds of troops were deployed to a nearby area in Patikul town, where the militants were believed to be holding the hostages.

Last week, over 3,000 soldiers were deployed to Jolo Island to step up security and patrols.

"We are ready for all-out operations against the kidnappers," said Colonel Allan Arrajado, commander of an army task force in Sulu province, which covers Jolo. "We will do our best so that their lives will not be endangered."

Four-million-euro ransom

It is thought the two German nationals - a 72-year-old man and his female 55-year-old partner - were kidnapped by the Islamist group Abu Sayyaf from their yacht on April 25 after it broke down traveling from the western Philippine province of Palawan to Borneo. The two have made multiple pleas via a radio station. On Wednesday, the man - who has been separated from his partner since the beginning of the month - told a local radio station that his captors had already dug his grave.

The group, which has links to al Qaeda, is demanding a ransom of 250 million pesos (4.4 million euros/$5.6 million) for the two German nationals. The release of the captives has also been made contingent on Germany's halting its military support for the US-led airstrikes against "Islamic State" (IS) fighters in Syria and Iraq. Although it was not immediately clear whether the group had links to the IS militia, Abu Sayyaf has been notorious for carrying out attacks and taking hostages in the Catholic-majority Philippines.

In line with German government policy, the foreign ministry has not released any details on possible negotiations. But in late September, it said Abu Sayyaf's tactics were "not an appropriate way to influence policy in Syria and Iraq."

"Threats are no appropriate way of influencing German foreign policy", a spokeswoman for the foreign ministry told Reuters news agency, adding that the ministry's crisis group was working on the case.

Rescue attempt could derail negotiations

"We know the general area where they are," Lieutenant General Rustico Guerrero, a Philippines' military commander, said. "We are aware of the demands and situation but this is all we can say. We don't want to put the lives of the hostages in jeopardy."

Another senior Philippine military officer in the region said negotiations with the group were on, but he did not know of any details.

"What I can tell you is we are ready to conduct operations, specifically for law enforcement operations, not necessarily for rescue," Rear Admiral Reynaldo Yoma told Reuters via telephone.

"If we plan to do a rescue, that might derail ongoing negotiations," he added.

sb/gb (dpa, Reuters)