NEW YORK (CNN/Money) - Robust sales in December helped close the 2003 holiday shopping season on a strong note, as reports indicated that redemption of gift cards and gift certificates helped merchants clear merchandise in the period's final week.

ShopperTrak said Monday in its weekly National Retail Sales Estimate (NRSE) that December sales increased 6 percent during the same period a year earlier.

While final numbers for the season are not yet in, the report said the strength of sales last month, combined with a strong November performance, could make the 2003 holiday season the most profitable since 1999.

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"Holiday sales came through in a big way late in the season," Michael Niemira, lead consultant for ShopperTrak wrote in the report. "Now it remains to be seen whether sales in the coming months fall off a bit given low remaining clearance inventories for many retailers. That however, should be taken as a positive sign for the industry, not a negative one."

ShopperTrak also reported that sales for the week ended Jan. 3 fell 35.1 percent over the previous week, reflecting the traditional "post-Christmas slowdown."

However, year-over-year sales for the same week jumped 46.5 percent on the back of the redemption of gift cards.

ShopperTrak's data are compiled from sales information of 30,000 retail outlets and malls around the country.

Separately, Visa USA said Tuesday that total holiday sales on its credit and debit cards came in at $115.6 billion, a 14.5 percent increase over the same period a year earlier.

Visa said overall retail expenditures on its cards for the period between Nov. 28 through Jan. 4 reached $49.4 billion, up 10.8 percent over the previous year.

U.S. chain-store sales slipped in the latest week after the late-December Christmas shopping blitz, according to a report Tuesday.

Sales fell 0.1 percent in the week ended Jan. 3, down from a gain of 2 percent the week before, the International Council of Shopping Centers and UBS said in its joint report.

Sales for the week increased 5.6 percent compared with the previous year, up from the previous week's 5.5 percent.

ICSC said December's comparable store sales should still register a gain between 4 percent and 4.5 percent.

The ICSC-UBS Weekly Chain Store Sales Snapshot is compiled from a group of major discount, department and chain stores across the country that report weekly results.  

-- from staff and wire reports