More than 100 container vessels are now waiting outside the port, where the average waiting time at berth last week went up 6.2 percent, to 18.2 hours, versus the previous week, according to shipping software provider CargoSmart.
CargoSmart said the fog started in early April and the congestion had worsened in the past few days, with few clues to when it might ease. Average delays for ships arriving at the port jumped more than 42 percent to 53 hours between April 16 and 18 from the start of the month, it said.
"We are making efforts to adjust and coordinate the entire port's resources and making every effort to provide customers with the fastest service," said a spokeswoman for Shanghai International Port Group (>> Shanghai International Port Group Co Ltd), which first flagged the situation last week in a statement on its website.
German container shipping line Hapag-Lloyd (>> Hapag-Lloyd AG) told customers in a notice on Thursday that it was monitoring the situation closely and working with the port to minimize delays.
Denmark's Maersk Line, part of A.P. Moller-Maersk (>> AP Moeller Maersk A/S), also confirmed that some of its vessels had been affected by the congestion.
"We are closely working with Chinese ports to address any operational challenges, and we have been able to maintain a very constructive dialogue with our terminal partners," a Beiijng-based spokeswoman for the company said.
A rejig in global container shipping alliances involving more than 10 ocean carriers, including Maersk, Hapag-Lloyd, COSCO Shipping (>> COSCO SHIPPING Holdings Co Ltd) <1919.HK> and Yang Ming Marine Transportation (>> Yang Ming Marine Transport Corp.) took effect on April 1, causing route adjustments and also putting into operation larger ships.
Executives also say demand for ocean freight has begun improving after years in the doldrums, fuelling an increase in volumes and rates.
Shanghai has been the world's busiest port by container volumes since 2010. It processed 37.1 million boxes last year, up 1.6 percent from the previous year, it said in a stock market statement last month.
(Reporting by Brenda Goh in SHANGHAI and Keith Wallis in LONDON; Editing by Clarence Fernandez)