Typhoon Kalmaegi has lashed Vietnam with fierce winds and torrential rains as it made landfall after leaving more than 100 people dead and dozens missing in the Philippines.

Three fishermen were reported missing on Thursday after their boat was swept away by strong waves on Ly Son, an island in Vietnam's Quang Ngai province. A search operation was launched but later suspended due to worsening weather, state media said.

Authorities said more than 537,000 people were evacuated, many by boat, as floodwaters rose and landslides loomed.

Vietnam's central provinces were already reeling from floods due to record-breaking rains. Kalmaegi is forecast to dump more than 600 millimetres of rain in some areas.

An unusually strong storm for the region in November, Kalmaegi packed sustained winds of about 183km/h with gusts reaching up to 220km/h over the South China Sea as it approached Vietnam.

Waves up to three metres high battered the coast in coastal cities like Danang, and strong winds uprooted trees in Dak Lak province. Many homes in Quy Nhon, also a coastal city, were left without power for hours.

The country's financial hub, Ho Chi Minh City, faced a heightened risk of severe floods. High tides were expected on the Saigon River, and authorities warned up to 100 millimetres of expected rainfall could inundate low-lying areas.

State of emergency in the Philippines

Across the central Philippines,Kalmaegi killed at least 114 people and left 127 missing in the deadliest natural disaster to hit the country this year. Philippine President Ferdinand Marcos Jnr declared a state of emergency on Thursday.

One of the mourners was Krizza Espra, who went Thursday to a mortuary where the bodies of her husband and three children were being held.

They were killed when the roof of their home collapsed in the town of Liloan in the Cebu district of the Philippines.

She said four others in her family — including her mother and aunt — remain missing.

"I hope someone can help speed up the search for their bodies before (they) decompose, we hope we can still recognise them," she said.

The typhoon displaced more than 560,000 villagers in the Philippines, including nearly 450,000 who were evacuated to emergency shelters, the Office of Civil Defence said.

Marcos'"state of national calamity" declaration allows the government to disburse emergency funds faster and prevent food hoarding and overpricing. Disaster-response officials warned that another tropical cyclone from the Pacific could strengthen into a super typhoon and batter the northern Philippines early next week.

Among the deaths attributed to Kalmaegi were six people who were killed when a Philippine air force helicopter crashed in the southern province of Agusan del Sur on Tuesday. The crew was on its way to provide humanitarian help to provinces battered by the typhoon, the military said. It did not give the cause of the crash.

Kalmaegi dumped about one-and-a-half months' worth of rainfall in just a day on Tuesday in metropolitan Cebu, state forecaster Benison Estareja said.

It set off flash floods and caused a river and other waterways to swell in Cebu city and outlying towns. The resulting flooding engulfed residential communities, forcing residents to climb onto their roofs, where they desperately pleaded to be rescued as floodwaters quickly rose, provincial officials said.

Rampaging floodwaters submerged or swept away scores of vehicles in Cebu's residential enclaves, in shocking scenes that were caught on camera by residents stranded on roofs.

Storm after storm

The Philippines is battered by about 20 typhoons and storms each year. The country also is often hit by earthquakes and has more than a dozen active volcanoes, making it one of the world's most disaster-prone countries.

Vietnam, which gets about a dozen typhoons and storms a year, has been pummelled by a relentless series this year, leaving little time to recover between disasters.

Typhoon Ragasa dumped torrential rain in late September, then Typhoon Bualoi slammed the central coast and Typhoon Matmo resulted in flooding in the north. Together, the three storms left more than 85 people dead or missing in two weeks, and caused an estimated $US1.36 billion ($2.16 billion) in damage.

Vietnam is among the world's most flood-prone countries, with nearly half its population living in high-risk areas. Scientists warn that a warming climate is intensifying storms and rainfall across Southeast Asia, making floods like this year's increasingly destructive and frequent.