The lone survivor, identified by Filipino police as an Indonesian teenager, was rescued in the Tawi-Tawi island group near the Malaysian state of Sabah on Monday.
"I condemn this outrageous and senseless killing as I condole with the families of the victims," Mrs. Arroyo said in a statement.
She ordered the Philippine National Police and the military to "block all possible exits out of the country and coordinate with Malaysian authorities" to set up a dragnet.
"This is another test of our regional covenant against terrorism," Mrs. Arroyo said.
Armed Forces Southern Command chief Lt. Gen. Roy Kyamko, however, said there were doubts as to the veracity of the claims of the lone survivor, Nonoy Arkusil, that the five other hostages were executed Monday night.
"Up to now, there is an air of doubts as to his claims. As the tactical interrogation went on, we are starting to doubt his claims if he was indeed a survivor of the said executions," Kyamko told reporters during a press conference with the President in Davao City.
Gunmen suspected of being Abu Sayyaf Islamist rebels operating in the southern Philippines seized six foreign workers from the Paradise Resort in the Malaysian section of neighboring Borneo island last Oct. 5.
Arkusil had told authorities that his companions were murdered by the gunmen believed led hy Ghalib Andang, alias Commander Robot.
It was unclear if any of the bodies had been recovered, and the Philippine government did not disclose their identities.
There are also conflicting reports of the nationalities of the hostages, with Malaysian authorities at the time of the abduction saying that they included three Filipinos and three Indonesians.
But Filipino authorities in the south said Arkusil spoke only Malaysian through an interpreter.
Asked if authorities on the ground have confirmed the killing of the five hostages, Presidential Spokesman Ignacio Bunye said: "I think the killings of the hostages have been confirmed."
Bunye however said it was not clear what group was responsible for the kidnapping, even as the police said it could be the Abu Sayyaf, a small group of Islamic militants involved in bombings and abductions in recent years.
Meanwhile in Manila, the government’s anti-kidnapping task force chief, Angelo Reyes, said the government would step up its campaign following a notable rise in abductions "which has scared away investors and tourists."
Average monthly kidnapping incidents rose to 8.3 from January to October, from 6.3 the previous year, with more than half of them taking place in Metro Manila, Reyes said.
The government has identified at least five "principal groups" behind the kidnappings, Reyes said. – AFP, Marichu Villanueva, Roel Pareño, Edith Regalado