U.S. Internet users viewed 13.1 billion online videos in February, down 12 percent from January, although viewing on Hulu grew 42 percent in the period, research firm comScore said on Tuesday.

The overall decline was mainly due to the fact February was a shorter month, with three fewer days than January, ComScore said in a statement released on Tuesday.

Google Inc's YouTube, which offers user-generated home videos along with professionally produced content, remained the No. 1 video site in the United States, although News Corp's Hulu experienced the largest increase in unique viewers compared with January, growing 42 percent to 34.7 million viewers in February, ComScore said.

Google sites accounted for 41 percent of the U.S. online video market, with 5.3 billion videos viewed and 99.4 million unique viewers with YouTube.com accounting for more than 99 percent of all videos viewed at Google properties.

News Corp's Fox Interactive Media ranked No. 2, representing 3.5 percent of the online video market, with 463 million videos viewed and 53.8 million unique visitors.

Coming in third was Yahoo! Inc sites, accounting for 2.7 percent of the market, with 353 million videos viewed and 41.7 million unique visitors.

ComScore noted Hulu rose two positions in the ranking to No. 4 for February, helped by a Super Bowl ad at the outset of the month. About 34.7 million unique visitors viewed 333 million videos on Hulu and related sites, accounting for 2.5 percent of the total online video market, ComScore said.

Rounding out the top five sites, was Microsoft Corp with 28.5 million unique viewers watching 259 million videos, accounting for about 2 percent of the market.

(Editing by Andre Grenon)