Heiko Maas said checks show that social networking sites have a patchy record of deleting posts that are considered illegal in Germany.

A two-month test conducted over the summer found that Facebook removed 46 percent of posts flagged by users, while Twitter removed just 1 percent. The figures increased to 91 percent for Facebook and 82 percent for Twitter when the companies were contacted directly or flagged by so-called privileged users.

Maas said the checks will continue until March.

(Read also: Facebook, Twitter, YouTube and Microsoft to remove hate speech across Europe)

"If it turns out that the removal rate remains so low, then we'll take legislative measures," he said.

Twitter declined to respond directly to Maas' comments, but cited its policy banning accounts whose primary purpose is to incite harm toward others. The company said it cooperates with law enforcement agencies and regularly publishes figures for how many information requests it gets from German authorities.

Facebook said it was working to improve the way it deals with hate speech. The company now has thousands of staff vetting reports and aims to respond within 24 hours.

Despite strict laws against incitement to hatred, harassment and anti-Semitism, Germany has seen a sharp rise in online hate speech.

Last year, a man was sentenced to two years and three months in prison for calling for the killing of Chancellor Angela Merkel.

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