A MID-AIR close call between two cargo planes taking off from Hong Kong's airport last Thursday has triggered an investigation, authorities said, removing air traffic control personnel from frontline duty.

The incident occurred on Thursday around 7 am 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, reports Hong Kong's The Standard.

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", Mr Chim told AFP.

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.