The incident occurred on Thursday around 7 am (Wednesday 2300 GMT) when two aircraft -- MNB380 bound for Turkmenistan and AZG625 for Baku -- took off from adjacent runways at the Hong Kong International Airport, according to the city's Civil Aviation Department.

They were told by air traffic control to maintain vertical separation but were unable to because their climbing rates varied from expectations, the department said.

The Boeing 777 freighter bound for Baku -- operated by Silk Way West Airlines -- issued a collision-related alert to its crew when it reached a spot around seven nautical miles northeast of the airport, then descended to 2,500 feet (762 metres).

The aviation agency said it had begun an investigation and removed "relevant air traffic control personnel" from frontline duties.

"Preliminary information indicates that the incident may involve non-compliance by ATC personnel with established procedures," the department said.

Warren Chim, deputy chairman of the Hong Kong Institution of Engineers' Aircraft Division, said it has long been standard for planes to come equipped with a traffic collision avoidance system (TCAS).

Available information "seems to show that the TCAS had functioned normally and prevented a mid-air collision", Chim told AFP.

The global aviation industry has seen shocking incidents in recent months, including a deadly crash between a US Army helicopter and a passenger jet in the skies above Washington.

In November, Hong Kong's airport completed its years-long expansion to bring its runway total from two to three, part of a bid to boost competitiveness.

The $18 billion project will enable the airport to handle 120 million passengers and 10 million tonnes of cargo annually by 2035, authorities said.