An unspecified number of air traffic control staff have been suspended over an incident involving two cargo planes that triggered an alert after take-off.

According to the Civil Aviation Department (CAD), the two aircraft were cleared to take off from Hong Kong International Airport on Thursday morning, heading to Turkmenistan and Azerbaijan.

One of the cargo planes was told by air traffic control to climb to 5,000 feet, and the other to 3,000 feet, to maintain separation.

"However, as the climbing rates of the two cargo flights varied from what the (air traffic control) officers had anticipated, the desired vertical separation was not achieved," a government statement read.

"During this time, both cargo flights followed standard departure procedures and maintained visual contact with each other."

The second plane then descended to 2,500 feet after its traffic collision avoidance system issued an alert to its crew.

Both aircraft continued to their destinations according to their flight plans.

The CAD said preliminary information showed the incident might be due to air traffic control staff failing to abide by take-off protocols, and those involved have been temporarily removed from frontline duties.

Secretary for Transport and Logistics Mable Chan on Friday said the department has been ordered to submit a report on the case.

"The report has to include, for example, procedures and staff related to the incident. I hope it can help prevent similar incidents in the future and allow us to do better," she said.