By Dominique Patton and Muyu Xu

BEIJING (Reuters) - Testing of imported food for the novel coronavirus is pushing capacity at some major Chinese ports to their limit, major shippers told customers this week, warning of additional fees and possible diversions to other ports.

China stepped up inspections of imported food last month after an outbreak of the coronavirus among people working at and visiting a major food market in Beijing. (Full Story)

"Import container pick-up activities have been severely impacted and as a result reefer plugs are highly utilized especially at the port of Yantian and Ningbo," said German shipping firm Hapag-Lloyd in a customer notice on Friday.

Reefers, or chilled containers used for meat and fresh produce, must be plugged in for the contents to be kept cool or frozen.

A person who answered the phone at one of Ningbo port's terminals referred Reuters to the port company's headquarters, which was closed after business hours. Nobody at Yantian port could be reached.

Though it has only found the virus in a handful of samples out of more than 200,000 tested so far, China continues to inspect cargoes and carry out tests on a large portion of arrivals. (Full Story)

Hapag-Lloyd warned that containers could be discharged at an alternative port and that cargo owners would be liable for additional costs.

Top container shipper Maersk MAERSKb.CO also told customers on Wednesday that it has been diverting cargo from Yantian port in the southern city Shenzhen because of limited reefer plugs.

The port is one of China's top ports for frozen meat.

"The terminal yard density for reefer units at Yantian has reached critical levels," it said in the letter seen by Reuters urging customers to book to nearby ports Nansha or Chiwan instead.

Any cargoes arriving in Yantian would face a congestion surcharge of $1,000 per container, it said.

"There are enough plugs for reefers at major ports and the offload process is also normal," said Ding Li, secretary general at the China Ports & Harbours Association.

Yantian port has added 35% more plugs recently to tackle the congestion, and plug utilisation rates at Ningbo port are at 65-70%, Ding said.

"The current bottleneck is insufficient inspection and disinfection capacity for the reefers at customs. But they are working on it," she said.

Responding on Saturday, Maersk said Chinese customs have added additional resources to accelerate customs clearance and that Yantian is now the only port still struggling to discharge all inbound reefers.