A retired Italian priest held hostage for six months by suspected militants left a Philippine hospital Saturday a day after his ordeal ended, looking frail though officials said he was in good health.

His face partly hidden by a red baseball cap, Rolando Del Torchio stroked his grey beard and waved briefly before getting into an ambulance that the military said delivered him to a chartered flight.

The authorities found him late Friday aboard a ferry docked on the remote island of Jolo, the main stronghold of the militant Abu Sayyaf group, some 950 kilometres (590 miles) south of Manila.

"The victim is emaciated. He has lost a lot of weight compared to what we saw in his old pictures," regional military spokesman Major Filemon Tan told reporters.

"He is okay otherwise."

The Italian government chartered the private plane for the former Catholic missionary, Tan told AFP, but Philippine officials would not say where he was taken. Calls to the Italian embassy in Manila went unanswered.

The Abu Sayyaf is a small group of militants infamous for kidnapping foreigners and demanding huge ransoms, as well as for being behind deadly bombings in Philipines.

Its leaders have in recent years pledged allegiance to the Islamic State group that controls vast swathes of Iraq and Syria.

Armed men posing as diners snatched the then 56-year-old Italian at his pizza restaurant in the southern city of Dipolog in October last year.

Tan said he did not know if any ransom was paid to win the Italian's release.

Del Torchio had worked as a missionary for the international organisation PIME in the south from 1998 before retiring in 2000 to set up his restaurant, colleagues told AFP shortly after he was abducted.

There are still 18 foreign hostages being held in the country -- 10 Indonesians, four Malaysians, two Canadians, a Norwegian and a Dutchman -- most of them thought to be held by the Abu Sayyaf.

The Abu Sayyaf last month posted a video of two Canadians, a Norwegian and a Filipina they kidnapped in September last year and set an April 8 deadline for ransom to be paid or the foreigners would be killed. The deadline passed Friday with no word on their fates.

The group beheaded a Malaysian hostage last year.

The Abu Sayyaf was established in the early 1990s with seed money from Osama bin Laden's Al Qaeda network.