China reopened a key terminal at the world's third-busiest cargo port on Wednesday, after a shutdown to control the coronavirus caused backlogs through supply chains — but disruptions at a major airport are now sending transport costs soaring.

Several cargo aircraft workers at Shanghai Pudong International Airport tested positive for the coronavirus in the past week, sparking a suspension in freight operations.

That has led to a spike in air freight rates of "around 30 per cent", SEKO Logistics told AFP.

The company added that it has rerouted cargo to different airlines and airports to avoid delays, after the current suspension starting last Friday.

"The closure is currently estimated to last around seven to 14 days," it said.

There have been flight cancellations at Pudong while in the industry, staff resignations following tighter quarantine rules have dragged handling times and created a "backlog for upcoming flights", said logistics company Ligentia in another recent statement.

The situation piles stress on already-stretched global supply chains.

The shipping network is still reeling from recent port closures, as exporters work to meet soaring demand for goods from Western consumers and comply with strict domestic virus controls.

The air cargo disruptions come shortly after a partial halt at the eastern Ningbo-Zhoushan port, starting two weeks ago when a worker at its Meishan terminal tested positive for the coronavirus.

The terminal handles a fifth of the container volume at Ningbo-Zhoushan and the hold-up forced ships to other Chinese ports, which were left facing their worst levels of congestion in seven years, reported Chinese media outlet Caixin this week.

The Ningbo-Zhoushan port handled almost 1.2 billion tonnes of goods in 2020.

But Ningbo authorities said late Tuesday that restrictions will be lifted from Wednesday morning.

Calling for a "resumption of work and production", authorities added that business should also prioritise loading and unloading stranded trucks, said a notice republished by state broadcaster CCTV.

Chinese port workers are routinely tested for COVID-19, and the affected worker had been fully vaccinated.

Similarly, the infected air cargo workers in Shanghai were fully vaccinated as well, according to state media reports.

China has been battling a resurgence of COVID-19 in recent weeks caused by the highly infectious Delta variant, but strict lockdowns and mass testing has helped push new case numbers back down.