MANILA, Sept. 25  -- The Philippine government vowed on Thursday to step up military operations in the country's restive south to go after Abu Sayyaf militants who threatened to kill one of two German hostages.

Defense Secretary Voltaire Gazmin said the military had deployed additional forces in Sulu province, a known stronghold of renegade Moro National Liberation Front forces.

"What the Abu Sayyaf (group) is doing is propaganda to force the government to give in to their demands. We will not be intimidated by their gestures and actions. We will continue to contain them," he said, confirming that a German couple was being held by the rogue group.

The minister said the government would not negotiate with Abu Sayyaf militants although he admitted that negotiations with the group and "other parties" were underway.

The group demanded a ransom of 250 million pesos (5.61 million U.S. dollars) in exchange for the freedom of Stephan Viktor Okonek, 71, and his wife, Herike Diesen, 55. They were kidnapped in April while island-hopping off the western province of Palawan.

Abu Sayyaf militants also called on the German government to stop supporting the ongoing U.S. campaign against "our Muslim brothers in Iraq and the Levant, and the Mujahideen of the Islamic State in particular."

Abu Sayyaf, a group of about 300 to 400 men infamous for their kidnapping, beheading and bombing activities, set an Oct. 10 deadline to meet their demands.