U.S. shoppers are still spending heavily online after a record-busting "Cyber Monday," research firm comScore Inc. said Sunday.

The firm, which tracks Web use, found shoppers spent nearly $6 billion online on Monday through Friday last week, a record.

On Cyber Monday itself, sales reached $1.25 billion, the biggest online shopping day in history. Online sales on Tuesday and Wednesday also broke $1 billion.

Cyber Monday sales topped $1 billion for the first time last year.

ComScore says online sales are up 15 percent to $18.7 billion in November and the first two days of December, compared with the same period last year.

The holiday shopping season can make up to 40 percent of retailers' annual revenue. This year's holiday shopping has risen with help from discounting and promotions.

Free shipping also appears to be a big draw, applying to 63 percent of sales, up from 52 percent a year ago.

"Consumers have come to expect free shipping during the holiday promotion periods, and retailers, in turn, have realized that they must offer this incentive," said comScore chairman Gian Fulgoni in a statement.

Online shopping accounts for between 8 and 10 percent of overall holiday spending, by various estimates. ComScore's spending figures exclude travel, auctions and large corporate purchases.

Spending on items including clothing, general merchandise, toys and electronics and in department stores, rose 4.7 percent to $125 billion in the Oct. 30 to Nov. 26 period, according to MasterCard Advisor's SpendingPulse. That includes online buying and spending in physical stores.