Filipinos have held vigils for the kidnapped Red Cross workers

A state of emergency has been imposed on a southern Philippine island where militants holding three aid workers have threatened to kill one of them.

The declaration puts all security forces on alert on Jolo island and curtails the movement of people.

The Abu Sayyaf rebels had threatened to behead one of the three Red Cross hostages by 1400 (0600 GMT) if security troops did not pull back.

But there has so far been no word on the fate of the hostages.

Swiss national Andreas Notter, Italian Eugenio Vagni and Filipina Mary Jean Lacaba were seized on 15 January, and have since been held in the jungles of Jolo.

The three aid workers were abducted after a visit to a local prison, where the International Committee of the Red Cross is funding a water project.

The head of the Philippine Red Cross has made a last-minute appeal to the militants to spare the hostages' lives.

"The whole family of the Red Cross prays for you and I'm proud of the way you've comported yourself," said Senator Richard Gordon in a tearful televised address.

"I'm sorry I should be stronger than you because I'm not in midst of the ordeal you're in now," he said.

At least 800 soldiers out of 1,000 have pulled back on Jolo island.

However the government says it cannot complete a full withdrawal as that would leave the island's civilian population exposed to militant attacks.

The Abu Sayyaf has a history of beheading captives.

In 2001, American Guillermo Sobero was killed after the government turned down attempts by the rebels to negotiate for hostages on the nearby island of Basilan.