BEIJING, April 2 -- Three leading Internet firms in China - Baidu, NetEase and MoMo - were investigated for spreading pornographic content in a campaign aimed to clean up the nation's cyberspace.

Police are investigating a column of NetEase, a popular web portal, for allegedly carrying digital novels that contain obscene or pornographic content, said a statement of the National Office Against Pornographic and Illegal Publications.

The statement said search engine giant Baidu was fined more than 210,000 yuan (about 33,800 U.S. dollars) in March for carrying online publications without permit and uploading obscene and pornographic novels onto its app.

MoMo, known for its dating app, was also fined 60,000 yuan in March for carrying pictures and videos that were lewd and pornographic on its online products, according to the statement.

The statement said authorities probed the three firms after being tipped off by the public.

The office said the three cases are the initial results of the campaign this year and it will impose harsher punishment for the firms that are repeated violators in next move.