A 190-metre-long cargo ship sank off Hong Kong Wednesday in a strong typhoon that whipped up towering waves and forced much of the city to shut down, after killing six in the Philippines.

Packing winds of up to 150 kilometers (90 miles) per hour at its centre, Typhoon Utor made landfall in mainland China at 3:50 pm (0750 GMT) in the southern city of Yangjiang, the official Xinhua news agency said.

More than 158,000 people had been relocated in southern China ahead of its arrival, Xinhua said, warning of heavy rains and strong winds.

The 21 crew of the bulk carrier Trans Summer abandoned ship as the vessel tipped on its side and sank 80 kilometres (50 miles) southwest of Hong Kong, triggering a rescue by two helicopter teams and a passing ship.

"The waves were 10 to 15 metres (30-50 feet) high. The wind was 90 kilometres per hour," Samuel Yip of the city's Flying Service told AFP.

"The air crew saw the ship was leaning to the left side by about 20 to 30 degrees and started to sink when it tilted almost 90 degrees."

Alan Loynd, the managing director of Branscombe Marine Consultants, said such ships were designed to withstand extreme weather so something must have gone wrong.

"If there's an engine breakdown, then the ship can get turned broadside on to the weather. Then the cargo could shift, forcing the ship over," he told AFP.

"Once the ship loses control in those sort of severe weather conditions it can quickly turn into a disaster."

A government spokeswoman said the Marine Department and China's Guangdong Maritime Safety Administration would hold a joint investigation.

Typhoon Utor left at least six people dead when it swept across the Philippines on Monday, flattening houses and bringing flash floods and landslides.

As it headed for mainland China, it barrelled past Hong Kong, where it forced the closure of financial markets, schools and businesses and disrupted hundreds of flights.

The Hong Kong Observatory hoisted a "Number 8" storm warning -- the third-highest level -- in the early morning.

As the typhoon skirted the city about 240 kilometres to the southwest, the observatory downgraded the warning in the afternoon to "Number 3", indicating strong winds.

Almost half of the day's flights were disrupted, with 118 cancelled and another 320 delayed.

Ferry services to outlying islands and mainland China were halted in the morning, stranding passengers at various terminals.

A government spokesman said six people were treated in public hospitals for storm-related injuries and there were six cases of minor flooding.

Hong Kong's streets were quiet for much of the day, with many workers staying home as businesses and schools were shuttered.

As the storm passed more people began venturing out, umbrellas hoisted, and the city returned to normal.

However, the Hong Kong stock exchange had already cancelled Wednesday's session and schools remained shut for the afternoon.

In the Philippines, the National Disaster Risk Reduction and Management Council said the death toll had risen to six, with five people still missing as rescuers rushed to reach areas isolated by the storm.

Two people were confirmed dead in the central island of Cebu after they were washed away by a flash flood while two fishermen drowned in the eastern town of Casiguran, the hardest hit by the storm.

In the northern Philippines one man was crushed by a landslide while another man drowned, the council said in a statement. Five people, mostly fishermen, were listed as missing.

Almost 42,000 people were still homeless in the Philippines after Utor toppled light structures, ripped the roofs off homes and buildings, and inundated farms, the council said. Some towns were cut off by landslides or fallen trees.

As the weather cleared, relief agencies sent workers and supplies to the ravaged towns while heavy equipment was deployed to clear the roads.

Hundreds of people die from the roughly 20 typhoons or tropical storms that strike the Philippines each year.

Utor hit land with wind gusts reaching 200 kilometres an hour early Monday, making it the strongest storm this year, according to the Philippine weather bureau.

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