China's top online portal Sina Corp has invested between $30-$40 million in Chinese online video site Tudou Holdings , a source familiar with the matter told Reuters on Monday.

Tudou, which listed on the Nasdaq last week, is China's second-largest online video site after Youku.com Inc .

The investment, which gives Sina around a 4 percent stake, may be a prelude to deeper cooperation between the two companies although those details have not yet been discussed, said the source who declined to be identified as the matter is not public.

We haven't discussed about more cooperation yet but it is a possibility in the future, the source said.

Tudou declined to comment while Sina could not be reached for comment.

Tudou, which raised $174 million in its Nasdaq public offering, saw its shares slip about 34 percent from its initial public offering price of $29, as Wall Street tumbled last week on euro zone debt uncertainties.

China is the world's largest Internet market, with more than 450 million Internet users, and online video firms are stepping up their fight for content to secure bigger market share to win advertisers.

Baidu Inc and Sohu also have popular online video offerings.

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