Same-store sales at Gap Inc, the US' largest speciality clothing retailer, fell by 6% in December as sales slumped at its Gap and Old Navy chains.

Net sales for the San Francisco based company were also down in the five-weeks to 5 January, to $2.20bn from $2.34bn in the same period last year.

By division, comparable store sales were down 9% at Gap North America and dropped 8% at Old Navy North America. Faring slightly better were the Banana Republic North America and International divisions, both of which saw sales fall 1%.

"We were pleased that merchandise margins in December were significantly above last year, which is consistent with our strategy of delivering earnings with healthy margins," said Sabrina Simmons, executive vice president of finance and acting chief financial officer of Gap.

"However, we did not sell through as much inventory as we anticipated, and we'll focus on clearing through remaining holiday product in January."

For the 48 weeks of the year-to-date, the company said net sales increased 1% to $14.83bn from $14.74bn.

Year-to-date comparable store sales decreased 5%, compared with a 7% decrease in the prior year.