Typhoon Yagi (locally known in the Philippines as "Enteng) weakened to a tropical depression on Sunday, after killing more than a dozen people, tearing roofs off buildings, sinking boats and triggering landslides across Vietnam.

The typhoon had left a trail of destruction and two dozen people dead across southern China and the Philippines before it ravaged Vietnam.

The storm killed 21 people and injured 229 in Vietnam, state media reported late Sunday.

Among the victims was a family of four killed after heavy rain caused a hillside to collapse onto a house in the mountainous Hoa Binh province of northern Vietnam, according to state media.

Since Friday, others have been killed in storm-related incidents, some crushed by falling trees or drifting boats, the defence ministry's disaster management agency.

On Sunday afternoon, six people, including a newborn baby and a one-year-old boy, were killed in a landslide in the Hoang Lien Son mountains of northwestern Vietnam.

The slide was triggered by heavy rains and high winds after Yagi made landfall on Saturday.

"We found the six bodies, including a one-year-old boy and a newborn, in the landslide," a local official from the Sapa people's committee, who asked not to be named, told AFP. 

"The rain was heavy, weakening the soil and triggering (the) landslide."

'No boat could stand'

While Vietnam's weather agency downgraded the storm on Sunday, several areas of the port city of Hai Phong were under half a metre (1.6 feet) of flood waters, and electricity was out, with power lines and electric poles damaged, according to AFP journalists.

At Ha Long Bay, a UNESCO World Heritage Site about 70 kilometres up the coast from the city, fishermen were in shock as they examined the damage Sunday morning.

The disaster management authority said 30 vessels sank at boat lock areas in coastal Quang Ninh province along Ha Long Bay after being pounded by strong wind and waves.

The typhoon also damaged nearly 3,300 houses, and more than 120,000 hectares of crops in the north of the country, the authority said.

Rooftops of buildings were blown off and motorbikes were left toppled over in piles of building debris, AFP journalists observed.

Pham Van Thanh, 51, a crew member of a tourist boat, said all the vessel's crew remained on board since Friday to prevent it from sinking.

"The wind was pushing from our back, with so much pressure that no boat could stand," Thanh told AFP.

"Then the first one sank. Then one after another," he said.

Blown off roofs

Bui Xuan Tinh said he lost both his home and business to the "destructive" typhoon, and would need to spend tens of thousands of dollars to repair his three wooden tourist boats after they sank in a lock on Tuan Chau island.

"I have been in this sea (and) ship business for decades and have never witnessed such a thing like yesterday," Tinh told AFP.

"Then I received a phone call from my kids at home saying our rooftop was blown off," he said. "I did not feel anything. What came to my mind was only 'Oh God, oh God'."

Before making landfall in Vietnam, Yagi tore through southern China and the Philippines, killing at least 24 people and injuring dozens of others.

Typhoons in the region are now forming closer to the coast, intensifying more rapidly, and staying over land for longer due to climate change, according to a study published in July.