People in the U.S. have steadily increased the amount of time they spend watching videos online and Google's YouTube remains by far their preferred video site, according to a study.

In July, almost 75 percent of U.S. Internet users watched videos online, up from 71.4 percent in March, according to comScore Networks.

The monthly time spent watching videos went up to an average of 181 minutes per viewer in July from 145 minutes per viewer in March, according to comScore.

People in the U.S. are also watching more video clips. In July, the average user watched 68 clips, up from 55 clips in March.

Overall, almost 134 million U.S. Internet users watched a little over 9 billion video clips in July, up from 126.6 million people and a little over 7 billion clips in March.

In July, Google ranked as the top provider of video clips, serving up 27 percent of the total -- almost 2.5 billion clips -- most of them via YouTube, comScore said.

Yahoo nabbed a distant second place, serving up 4.3 percent of the clips. Fox Interactive Media, the News Corp. Internet division that includes MySpace.com, came in third with 3.3 percent. Viacom (3.1 percent) and Disney (2 percent) rounded out the top five.

Google also ranked first in July in unique video viewers with almost 68 million, followed by Fox Interactive (35.8 million), Yahoo (35.3 million), Time Warner (26.6 million), and Viacom (22.6 million), comScore said.

After years of unfulfilled promises, online video has taken off in a big way in the past year and a half, rapidly accelerating its momentum across a wide variety of sites.

The revolution is widely credited to YouTube and its video-sharing format, but now companies are prominently featuring videos in portals, news sites, blogs, social networks, online stores, and film and television industry sites.

YouTube's popularity can be attributed to several factors, including its ease of use for both viewers and uploaders; a strong community of millions of people who submit, view, and share personal videos; and an abundance of commercial clips from movies and television.

It is this last component of its popularity -- the commercial clips -- that have put YouTube at the center of the Internet industry's struggle with video copyright protection because many of those videos are copied and uploaded without their owners' permission. A landmark legal action is currently under way, as Viacom pursues a copyright-infringement lawsuit against Google over the unauthorized appearance of its clips on YouTube.

In addition to copyright, other technical and operational issues are in flux, such as the options for generating revenue from video content, the best ways to use video for online advertising, and the different alternatives of delivering the clips to viewers.

Another area of interest are video search engines, as well as alternate devices for online video beyond the PC, from small ones like cell phones to large ones like living-room home entertainment centers.

For example, currently the Web seems a vehicle primarily for short video clips as evidenced by comScore's finding that the average video clip duration in July was 2.7 minutes.