About 72 vessels were spotted off Qingdao, almost double the amount at the end of February, according to shipping data compiled by Bloomberg. The increased delays there and in other parts of China are expected to push up freight rates.

While there is usually a build-up of vessels seeking to enter China following Chinese New Year, volumes this year have increased by lockdowns aimed at curbing Covid.

"The virus outbreaks are sprouting up in different parts of China, and lockdown measures do not seem as effective because the transmissibility of the new variants are higher," said Salmon Aidan Lee, head of polyesters at energy consultancy Wood Mackenzie.

"That leaves us with a situation that has worsened in the past few days at Qingdao," he said, adding that he expects freight rates to rise because of increasing delays.

There's also a growing backlog of vessels off the ports of Shanghai, Ningbo and Zhoushan. There were 262 ships counted there, up from 243 a week ago. However the situation off Shenzhen and Hong Kong has eased, dropping to 162 vessels from 208 on March 7.

The rapid spread of the omicron variant in China is increasing the pressure on already strained supply chains reeling from Russia's invasion of Ukraine. Shipping lines like AP Moller-Maersk have cancelled services to Russia and halted some rail shipments from China into Europe.