Online spending in the United States rose 12 percent to $27.5 billion till date this holiday shopping season compared with the year-ago levels, according to research firm comScore.

With less than a week to go for Christmas, Free Shipping surge remained strong during the final online sales push, the research firm said.

Free Shipping Day achieved a 61 percent rise versus the same day of last year, with more than 1,500 merchants offered free shipping, comScore said.

From November 1 to December 17, four individual days surpassed $900 million, led by Green Monday on December 13 with $954 million and Free Shipping Day on December 17 with $942 million, according to comScore.

Free Shipping Day punctuated an exceptional week in which consumers spent more than $5.5 billion online, representing a 14 percent increase from last year, comScore Chairman Gian Fulgoni said.

We have likely already witnessed the peak spending day of the year, which means that Cyber Monday should emerge as the season's heaviest online spending day for the first time in history, said Fulgoni.

comScore said purchases of handheld devices such as Apple iPads and e-readers and laptop computers drove a 25 percent growth in computer hardware.