Empty containers, crucial to Asian exporters, are getting stuck in the port of Rotterdam as a growing backlog of undelivered goods at the European hub forces shipping lines to prioritize loading full containers.

Europe's biggest port has faced a flood of goods and empty containers arriving from other terminals on the continent, shipping experts said. This coincided with a drop in the number of ships sailing from Europe to China after Shanghai authorities locked down the city in March.

"The terminals are allowing shipping lines only limited capacity for empty containers and are focusing on the transport of full containers," said Kuehne + Nagel International. "Containers are piling up at terminals, and more and more containers cannot be returned on departure, including those from Asia," they added.

Although Shanghai authorities lifted restrictions this month, ship shortages and delays at European ports are forcing export goods to compete with empty containers for space on ships bound for China, threatening with fueling a new wave of disruption to Asia, the logistics companies said.

Factories in China require empty containers to make up for lost orders in recent months and will be hampered by vessel and equipment shortages, they said. The volume of containerized goods trucked into Shanghai from nearby Chinese provinces has recovered to 71% of the levels seen on March 12, according to data from FourKites.

Supply chains have been struggling to recover from the Zero Covid policy enforced by Chinese authorities and Russia's invasion of Ukraine, prompting companies to shun overland rail that might otherwise have eased pressure on clogged ports.

High levels of congestion in Rotterdam prompted Maersk to inform customers last week that it would relocate containers with unclaimed goods out of the port.

The situation has meant longer waiting times for ships in all major northern European ports, Kuehne + Nagel said.

About 60% of the ships that traveled from Europe to Asia in April and May this year were delayed, with a total of 13 waiting to dock in Rotterdam on June 17.