LOS ANGELES (Reuters) - Imports of U.S. container cargo in August jumped 12.9% from a year ago as a summer volume surge delayed cargo at major ports and anxiety builds over a threatened longshore worker strike on the East Coast and Gulf of Mexico on Oct. 1, trade data provider Descartes Systems Group ( DSGX ) said on Tuesday.

U.S. seaports processed almost 2.5 million 20-foot equivalent units in August. That was 3% less than in July, when imports hit a 26-month high. Volume above 2.4 million TEU created bottlenecks and cargo backups in the early days of the pandemic, according to Descartes, a supply-chain software provider.

The International Longshoremen's Association represents 45,000 dock workers at three dozen U.S. ports from Maine to Texas and handles almost half of the nation's ocean trade.

ILA said last week its members are prepared to hit the streets if they do not have a new contract with the United States Maritime Alliance (USMX) employer group in place when the current six-year agreement expires on Sept. 30.

The two sides are at an impasse over issues ranging from wages and benefits to automation.

A.P. Moller-Maersk, the second-largest container carrier and a member of USMX, told customers on Tuesday that the possibility of a strike at ports including New York/New Jersey, Houston and Savannah, Georgia, increases each day without a settled contract in hand.

"Should a general work stoppage occur on the U.S. Gulf and East Coasts, even a one-week shutdown could take 4-6 weeks to recover from, with significant backlogs and delays compounding with each passing day," Maersk said.

A recent trend by U.S. retailers to push forward holiday promotions suggests they have been rushing in seasonal goods to mitigate the risk of cargo getting stranded during a strike.

Walmart ( WMT ) and Target ( TGT ), two of the largest container shipping customers, held early back-to-school sales to fend off competition from Amazon.com's Prime Day. The late timing of Thanksgiving this year is shortening the Christmas shopping season and boosting the likelihood of early promotions.

An increase in industrial-sector shipments also amplified the early rise in incoming volume, trade researchers said.