Photos and live video footage from local media show the plane's fuselage partly submerged in water, with large cracks visible and its emergency evacuation slide extended.

The Boeing 744 cargo plane had arrived from the United Arab Emirates at Hong Kong International Airport, which is one of the busiest in the world for air cargo.

The plane "deviated from the North Runway after landing and ditched into the sea", Hong Kong's Civil Aviation Department said.

"Preliminary information indicates that the four crew members on board were rescued and sent to hospital, while two ground staff were affected and fell into the sea," the department added.

The accident, which took place at around 3:50 am local time (1950 GMT Sunday), left the aircraft's nose resting above water with its tail broken off.

The plane struck a ground vehicle, which also fell into the sea, authorities said.

A 30-year-old man aboard the ground vehicle was confirmed dead at the scene while another, aged 41, died after being rushed to hospital.

The airport's north runway was temporarily closed on Monday, while the other two runways remained operational.

A spokesman for the Transport and Logistics Bureau expressed deep concern over the incident, adding that the Air Accident Investigation Authority will "actively investigate the cause of the accident".

Helicopters from the Government Flying Service and vessels from the Fire Services Department have been deployed, according to media reports.

A dozen cargo flights have been cancelled throughout Monday, but passenger flights appeared to be unaffected, the South China Morning Post reported.

Hong Kong began flight operations on its third runway last November, with the city's airport being among the busiest in the world.

The expansion project cost HK$142 billion ($18 billion) and took eight years of construction, with officials saying it would keep the city's airport competitive as an aviation hub.