At least 26 people died on Tuesday when their bus plunged into a deep ravine in a northern Philippine mountain town, in one of the country's deadliest road accidents in years, officials said.

The rest of the more than 50 passengers were retrieved by rescuers who struggled with ropes to descend down the 24-metre-deep ravine to reach the wreckage in Nueva Ecija province's Carranglan town, said disaster-response officer Mark Raymond Cano.

Many of the passengers had serious injuries and were taken to hospitals, with parents being separated from their children in the confusion, officials said.

"One child here sat beside her mother. The mother was dead," said Dr. Napoleon Obana of the Veterans Regional Hospital, where some of the injured were taken.

TV footage showed a line of rescuers and villagers, some holding on to a rope to climb up on a grassy slope with a victim on a stretcher. A throng of villagers waited in a clearing above them to help pull the victim to safety.

Cano said a preliminary investigation showed that the bus "lost its brakes and plunged down the ravine".

The spot along the downhill road beside the ravine is accident-prone. Tuesday's accident was the sixth in the area in recent years, a local village official told DZMM radio.

The impact from the plunge ripped the bus, which could no longer be distinguished at the bottom of the ravine, said Nueva Ecija's police chief, Antonio Yarra. He said investigators could not yet talk to the still-stunned injured survivors to get more details on the accident

Carranglan police chief Robert de Guzman said the bus from northern Isabela province was on its way to Ilocos Sur, a tobacco- and rice-growing region also in the mountainous northern Philippines, when it crashed shortly before noon in Nueva Ecija, about 200 kilometres north of Manila.

Poorly maintained passenger buses, inadequate road safety features and weak enforcement of local transport laws have been blamed for many vehicular accidents in the Philippines.

In February, a sightseeing bus carrying college students on a camping trip lost its brakes as it sped down a steep downhill road before smashing into a concrete electrical post in Tanay town in Rizal province east of Manila, killing 15 people.