Demand for goods from Chinese e-commerce retailers such as Shein and Temu, which offer fast delivery times, are responsible for a spike in air cargo rates from China to the U.S. that is atypical for this time of year.

According to Nikkei Asia, the air cargo market is already under strain because of limited flights, and Shein and PDD Holdings' Temu ship goods directly from Chinese factories to U.S. consumers using air cargo.

Read More: France Cracks Down on Temu, Shein Amid Concerns Over Alleged Forced Labor

Air cargo rates from China to the U.S. are currently up 14% compared with the same period last year, generally known as the off-peak season. The global average was down 8% in the same period, and rates for cargo from the U.S. to China were down 29%.

A U.S. congressional report from June last year estimated that Shein and Temu were responsible for almost 600,000 packages per day arriving in the U.S.