Manila - A Singaporean terrorist suspect has been working with Muslim militants holding captive three Red Cross workers on a southern Philippine island, officials said Thursday.

The Singaporean suspect has been with the Muslim Abu Sayyaf rebels since the start of the hostage crisis on Jolo island, 1,000 kilometres south of Manila, more than two months ago.

"We confirmed that a Singaporean terrorist is with the group," National Security Adviser Norberto Gonzales said. "We identified the Singaporean in one of the cellular phone conversations."

Gonzales said the involvement of the Singaporean suspect "was part of the infiltration of the Jemaah Islamiyah (JI)" in the southern Philippines.

Interior Secretary Ronaldo Puno described the Singaporean as "one of those international terrorists that are under the protection of the Abu Sayyaf."

Security experts have warned that JI operatives were hiding and training in the southern Philippines in cooperation with local militants, such as the Abu Sayyaf.

Puno did not name the Singaporean suspect, but added that the foreigner was believed to have interpreted for the Abu Sayyaf rebels in contacts with the families of the hostages.

The rebels had threatened to behead one of the hostages - Swiss Andreas Notter, Italian Eugenio Vagni and Filipino Mary Jean Lacaba - unless government forces withdrew from a large portion of Jolo.

But authorities said the kidnappers did not carry out the threat and the hostages were still alive after a deadline to meet their demand expired on Tuesday.

Senator Richard Gordon, chairman of the Philippine National Red Cross, said he still has not received any proof from the Abu Sayyaf that the hostages were still alive.

"It's the second day after the deadline, and there's still no communication from them," he said. "We have no proof of life. We cannot rely on someone saying they saw the hostages, that's hearsay."

Gordon said mobile communication networks on Jolo have been down since Governor Abdusakur Tan placed the island under emergency rule on Tuesday.

A curfew was imposed and checkpoints set up around the island, while soldiers, policemen and militiamen were deployed to step up security.

The hostages, personnel of the International Committee of the Red Cross, were abducted on January 15 after visiting the provincial jail in Jolo to oversee a water and sanitation project.

Alain Aeschlimann, the committee's head of operations for East Asia, South-East Asia and the Pacific, urged the kidnappers to free the hostages "safe and sound."

"We are extremely concerned about the fate and safety" of the hostages, he said. "We continue to hope that the worst did not happen and will not happen."

Aeschlimann also urged Philippine authorities "to do everything in their power to save the hostages' lives and to avoid taking any action that could put the [Red Cross] staff at risk." (dpa)