U.S. consumers' visits to Internet shopping sites rose 20 percent last week from a year earlier as many online shoppers took advantage of shipping guarantees to buy items such as jewelry and video games for Christmas.

Shoppers made 551.6 million visits for the week through Dec. 17, up from 459.5 million visits a year earlier, New York-based research firm Nielsen/NetRatings said in a statement yesterday. Three of the year's four biggest online shopping days took place last week.

Traffic rose as more retailers streamlined shipping operations to guarantee delivery before the holiday.

The growth in retailers offering both online and on-site sales outpaced online-only companies, Nielsen/NetRatings said.

"Retailers as well as their shipping partners have gotten very good at being on time and as efficient as they possibly can," said Patti Freeman Evans, an analyst at Jupiter Research in New York.

Internet sales for the 50 days through Dec. 20 increased 26 percent to $21.7 billion, helped by a surge of last-minute online shopping, Reston, Va.-based ComScore said in a separate statement yesterday.

Sales for the three days through Dec. 20 surged 35 percent, the research firm said. Earlier this week, ComScore boosted its holiday spending growth forecast to 25 percent from 24 percent.

Online purchases of jewelry and video games during the holiday season each surged more than 60 percent from the corresponding period a year earlier, while event-ticket and video-game console purchases increased more than 50 percent, ComScore said today.

Online spending has climbed while overall U.S. retail sales have trailed some forecasts because consumers delayed their shopping until closer to the holiday.

Retail sales for the week through Dec. 16 rose 3.2 percent, trailing a forecast of 5 percent for November and December, Chicago-based ShopperTrak RCT Corp. said earlier this week.

Guarantees that products purchased as late as Dec. 18 would be delivered in time for Christmas helped to push online sales higher, ComScore said.

Sales at stores open at least a year rose 2.4 percent in the week through Dec. 16 from a year earlier, New York-based trade group International Council of Shopping Centers and UBS Securities LLC said Tuesday.

This was the slowest pace in a month as consumers put off their holiday shopping, and mild weather limited sales of hats and coats.