Import volumes at major retail container ports in the US are expected to grow 9.1% year-on-year in October, despite concerns over the government shutdown ahead of the holiday season.

The Global Port Tracker report from the National Retail Federation (NRF) and Hackett Associates reflects merchandise ordered months before the shutdown as retailers planned for the holiday season.

"With the holidays nearly here, retailers are making sure their shelves are well-stocked," NRF vice president for supply chain and customs policy Jonathan Gold said.

"Cargo is continuing to move through the ports but the government shutdown has left some agencies short-handed, so NRF will monitor the situation closely as the holidays approach."

The forecast comes as NRF expects holiday sales will grow 3.9% over last year to reach US$602.1bn.

US Customs and Border Protection (CBP) has furloughed 6,000 workers because of the government shutdown, which started last week, but acting commissioner Thomas Winkowski said the impact at the docks should be "minimal" since ports will remain open, with inspectors continuing to work and process cargo.

But other government agencies that have a role in clearing cargo at the ports have not remained as staffed as CBP, leaving retailers cautious.

Cargo import numbers do not correlate directly with sales because they count only the number of cargo containers, not the value of the merchandise inside them.

US ports followed by Global Port Tracker handled 1.48m Twenty-foot Equivalent Units (TEU) in August, the latest month for which after-the-fact numbers are available - up 3.8 % on last August and 2.5% on July.

September was estimated at 1.47m TEU, up 4.9% from last year. October is forecast at 1.46m TEU, up 9.1%; November at 1.33m TEU, up 3.4%; and December at 1.31m TEU, up 1.8 %.

In addition, January 2014 is forecast at 1.35m TEU, up 2.9% from January this year, and February at 1.18m TEU, down 8.1%.

The first six months of 2013 was 7.8m TEU, up 1.2% from the first half of 2012, while the total for 2013 is forecast at 16.3m TEU, up 2.7% from last year's 15.8m TEU.

"The supply-and-demand balance dictates pricing," Hackett Associates founder Ben Hackett said. "This has left the carriers to find ways to cut costs as a means to better financial results. Using larger ships is one solution, and larger alliances as a means to managing capacity is another."