MANILA (3rd UPDATE) - Four personnel of the Philippine Air Force (PAF) were killed after a military helicopter crashed in Basilan province on Wednesday afternoon amid bad weather, authorities said.

The casualties include 2 pilots and 2 crew members assigned at PAF's 505th Search and Rescue Group, Air Force spokesperson Lt. Col. Aristides Galang Jr. told TeleRadyo.

The Sikorsky helicopter took off from Zamboanga City around 12:30 p.m., bound for Jolo, Sulu for a medical evacuation mission when it crashed in Barangay Upper Manggas in Lantawan town around an hour later, Galang said.

There was a strong gust of wind followed by a heavy downpour before the incident, according to Lantawan municipal administrator Tahira Ismael who confirmed the aircraft crashed in the town's jurisdiction.

The bodies of the four on board, one of whom a female, were recovered from the crash site.

In a separate interview, Galang said the aircraft was supposed to transport to Zamboanga City some personnel who were wounded during the Aug. 24 suicide bombings in Jolo.

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The wounded personnel initially underwent treatment in Sulu.

Galang said investigators are still conducting probe to determine what caused the helicopter to crash.

Meantime, he said all Sikorsky helicopters of the Air Force have been grounded, pending investigation.

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In a statement, PAF extended its deepest condolences to the families of the personnel killed in the crash.

"With utmost respect. the PAF headed by its Commanding General. Lt. Gen. Allen Paredes extends its deepest and most sincere condolences to the bereaved families," PAF said.

The bereaved families were also immediately informed of the incident, it added.

Due to the incident, PAF said it will be conducting prompt maintenance of all S-76A aircraft.

A team will also be sent to the area to conduct further investigation.

— Report from Queenie Casimiro and Chiara Zambrano, ABS-CBN News