Major shipping companies have warned clients of worsening congestion at Shenzen's Yantian port in southern China following the discovery of several asymptomatic cases of COVID-19 in the city.

Yantian International Container Terminal (YICT), one of China's busiest container ports with an annual handling volume of more than 13 million twenty-foot equivalent units (TEU), has imposed stringent disinfection and quarantine measures since May 21 when the virus was discovered among port staff.

More than 40 container ships were anchored in open water outside the terminal, Refinitiv ship tracking data showed on Thursday.

Top international container lines said they would be skipping some port calls to Yantian to ease the pressure, while shipping sources added that some vessels may be re-routed to other destinations.