AMERICAN import volumes through major US container ports are expected to grow 9.1 per cent year on year in October, despite concerns over the federal 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 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 per cent over last year to reach US$602.1 billion.

US Customs and Border Protection (CBP) has laid off 6,000 workers because of the government shutdown, but acting commissioner Thomas Winkowski said the impact on 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 1.48 million TEU in August, the latest month for which after-the-fact numbers are available - up 3.8 per cent on last August and 2.5 per cent on July.

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

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

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