U.S. retailers who saw strong online sales as the holiday selling season kicked off over the weekend aim to cash in further on that momentum with Cyber Monday, one of the biggest days for Web commerce.

The term Cyber Monday was coined five years ago for the day many people return to work after U.S. Thanksgiving Day and make online gift purchases on their computers.

More recently, retailers have been increasingly offering online deals on Thanksgiving itself and through the holiday weekend, as consumers regularly shop on the Internet.

This year, online spending on last Thursday's Thanksgiving Day rose 28% from a year ago to $407 million, according to analytics firm comScore. ComScore has forecast overall online sales for the holiday season, when most shopping is done between Thanksgiving and Christmas on Dec. 25, to rise 11% to $32.4 billion.

Those early returns helped boost shares of Web commerce companies, with industry leader Amazon.com Inc rising 1.6% to $180.01 after hitting an all-time high of $181.84 earlier in the session.

Overstock.com shares were up 2.1% while Blue Nile was up 3.3% compared with a decline of 0.9% in the Standard & Poor's 500 Index.

According to the National Retail Federation, about 33.6% of people who went shopping during the four-day holiday weekend bought goods online, up 15% from a year ago.

The amount they spent is up as well. The trade group estimates that out of the $365.34 spent by the average consumer this weekend, $121.67, or 33.3%, was spent online. That is up from 30.2 % last year.

At the same time, the early deals on offer could eventually chip away at "Cyber Monday" as an event on the calendar, some analysts said.

"At this point, Cyber Monday could be relegated to the scrap heap in terms of importance for retailers," said Wall Street Strategies analyst Brian Sozzi in a research note.

Traditional retailers are also competing more fiercely for online sales, with Wal-Mart Stores Inc (NYSE:WMT) going head-to-head with Amazon on free shipping of merchandise during the holidays.