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 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.

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

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

(Reporting by Melanie Lee; Editing by Jacqueline Wong)