NEW YORK (Reuters) - U.S. online spending rose 4 percent to $29.1 billion in November and December with sales showing particular vigor over the Black Friday weekend and the week leading up to Christmas, according to data released on Wednesday by comScore.

The performance beat comScore's expectations and was fueled in part by guaranteed shipping, effective discounting and poor weather that forced shoppers online, comScore Chairman Gian Fulgoni sai in a statement.

Fulgoni said the results were encouraging but warned that high unemployment and consumer debt levels could curb consumer spending.

Sales were up 11 percent on Black Friday compared to the corresponding shopping day a year earlier and rose 13 percent on the weekend immediately preceding Christmas, when a major snowstorm slammed the East Coast.

comScore said that jewelry and watches saw the biggest increase of any category, rising 20 percent compared to a year earlier when sales plummeted 29 percent.

Consumer electronics also showed strong growth, rising 15 percent in November and December, fueled by sales of heavily discounted flat screen television sets.

December 15 marked the first time online sales surpassed $900 million in a single day, reaching $913 million, according to comScore.

Last week, comScore had said overall U.S. online spending had risen 5 percent over the holidays, but that was limited to the period from November 1 to Christmas Eve.

(Reporting by Phil Wahba)