Containers full of frozen food and chemicals are piling up at China’s biggest port in Shanghai as a Covid lockdown in the city and compulsory virus testing means truckers can’t get to the docks to pick up boxes, Bloomberg reports.

A shortage of trucks to haul containers from the port is impeding the clearance of imports, Ocean Network Express said in a customer advisory Wednesday.

While the port is operating normally, there are a “critically high” number of refrigerated containers and items classified as dangerous goods piled up at two storage yards, meaning some ships carrying those types of cargo may not be able to unload any more boxes at the port, it said.

Shanghai is now the epicenter of China’s worst Covid outbreak in two years, with more than 21,000 cases reported just on Thursday.

The shortage of trucks is also hitting companies in the city, which have been able to continue working through the lockdown, with chip giant Semiconductor Manufacturing International Corp. struggling to secure trucks to ship out finished goods.

Truckers form a crucial component of supply chains in China, moving raw materials from coastal ports to factories further inland. The backlog is likely contributing to growing ship queues off China, threatening even more delays and higher freight rates in coming months.