NEW YORK: The amount of time US web users spend watching online video rose in May 2011, with Hulu maintaining its position as the nation's number one provider of video ads.

In all, 176.4m users viewed an average of 951 minutes of online video content over the month, according to comScore. April's totals were 172m and 897 minutes respectively.

Online video ad exposures rose still more sharply, from 3.8m ads viewed and 30.3 ads per viewer in April to 4.6m and 33.7 in May.

Hulu was the top ad venue for both months, accounting for 1.3m ad views in May. This is mainly due to the fact that viewers use Hulu to watch long-form content - the number of ads viewed per user on Hulu was 47.6, twice as much as any other site in the top 10.

comScore also found the duration of the average online content video viewed was 5.2 minutes in May, while the average online video ad lasted 0.4 minutes. Just over one in ten (12.6%) of online videos viewed were ads.

Google-owned sites, including market leader YouTube, remain by far the most-popular online video platforms on the web. Collectively, they attracted 147.1m unique viewers in May. VEVO, a music video specialist, was second on 60.3m.

Video has become an increasingly important part of the digital marketing repertoire with advertisers attempting to find ways of engaging web users.

A report from eMarketer released last month showed that a slim majority (57%) of agencies believe online video is "more effective" than online display ads.

Separate eMarketer forecasts indicate that online video adspend will increase by 38.6% in 2011. For the same period, Warc forecast a 12.4% increase for US internet adspend.

Data sourced from comScore/eMarketer/Warc; additional content by Warc staff