The Philippine military came under increased pressure on Tuesday to rescue more than 20 foreign hostages after their Muslim extremist captors beheaded a Canadian man, but troops face a dilemma in how to succeed and also ensure the safety of the remaining captives.

Abu Sayyaf gunmen beheaded John Ridsdel on Monday in the southern densely forested province of Sulu, sparking condemnations and prompting Canadian Prime Minister Justin Trudeau to pledge to help the Philippines pursue the extremists behind the "heinous act".

Ridsdel's head, which was placed in a plastic bag, was dumped by motorcycle-riding militants on Monday night in Jolo town in impoverished Sulu, about 950 kilometers south of Manila, where the Abu Sayyaf and allied gunmen are believed to be holding 22 foreign hostages from six countries.

It's a politically-sensitive time to carry out major offensives at the height of campaigning in a closely-fought race by four contenders in May 9 presidential elections.

President Benigno Aquino III and opposition politicians have had differences over the handling of the Muslim insurgencies and the poverty and social ills that foster it.

'Full force of the law'

"The pressure on the armed forces is really immense," analyst Julkipli Wadi said. "The approach is still conventional and largely detached from the overall political question."

The Philippine military and police said "there will be no letup" in the effort to combat the militants and find the hostages, even though they have had little success in safely securing their freedom. Many hostages were believed to have been released due to huge ransom payments.

"The full force of the law will be used to bring these criminals to justice," they said in a joint statement.

About 2,000 military personnel, backed by Huey and MG520 rocket-firing helicopters and artillery, were involved in the manhunt for the militants, who were believed to be massing in Sulu's mountainous Patikul town, military officials said.

Among the 22 hostages are 14 Indonesians who were kidnapped from tugboats in two separate incidences. Indonesian President Joko "Jokowi" Widodo said on Tuesday that his country will host talks with Malaysia and the Philippines to boost maritime security following those hijackings.

In past militant videos posted online, Ridsdel and fellow Canadian Robert Hall, Norwegian Kjartan Sekkingstad and Filipino Marites Flor were shown sitting in a clearing with heavily armed militants standing behind them.