Google will this week take its "first step into TV advertising" by offering online video commercials. The move could increase the already intense pressure on the traditional television networks from the online search group.

The video ads will appear first on websites in America, Canada and Japan as static screenshots in small television-screen like boxes.

They will not be placed on Google pages but will appear on sites that are signed up to Google's AdSense advertising network. When a viewer clicks on an ad, it will begin to play.

Google says the service will give both viewers and advertisers unprecedented control over where TV-style ads appear and which are watched.

"Unlike some intrusive advertising, users will have complete control," Google's InsideAdwords blog said.

"When a page loads, only a static image will be visible; the video will not start playing until the user initiates it. He or she will be able to advance the video, pause it, adjust the volume or click through to the advertiser's site."

The ads will be triggered to appear on pages covered by Google's AdSense system after advertisers decide to target particular audiences through variables such as certain keywords that appear on a page.

Greg Sterling, an analyst with Sterling Market Intelligence in California, said on his blog that this week's initiative by Google "starts to bring the promise of the internet's precision targeting capabilities to TV advertising, and, in a way, this is Google's first step into TV".

The click-to-play adverts will also allow advertisers to monitor the effectiveness of campaigns by measuring how long and how often users watch them.

The service, which will later be rolled out across other territories including the UK, is designed to diversify Google away from the simple text-based ads that account for the bulk of its revenues.

While online advertising is growing rapidly, it still only accounts for a small portion of total marketing spending. Broadcast and cable TV advertising in the United States will be worth approximately $38 billion this year , according to Universal McCann. This is a prize on which online groups including Google and rivals Yahoo and Microsoft are focused.

For some time Wall Street has been talking up Google's entry into video. Earlier this year, analysts at UBS said they were "particularly excited about the potential of video ads and other rich media possibilities". Morgan Stanley’s West Coast team announced that "the thing we were most excited about" was the launch of Google Video, the company's video search system.

Mr Sterling said that the online video ad service would also give "a much-needed boost to Google's contextual network", which is generally seen as less effective than its search network and is regarded as attracting more click fraud - where people click on online adverts to dupe advertisers into thinking that adverts are attracting more interest from consumers than they actually are.

"In addition, it suddenly gives Google a real product for brand marketers, something the company has wanted but hasn't really had until now," he added.

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