The German Football Federation (DFB) has opened an investigation into Borussia Dortmund's goalkeeper Roman Weidenfeller over alleged racist comments.

Schalke 04's Ghana-born striker Gerald Asamoah claims Weidenfeller made the comments before Asamoah scored in his club's 4-1 Bundesliga win on Saturday.

It is alleged that in the 51st minute of the match, Weidenfeller called Asamoah, the first black player to be selected for the German national team, a "black pig".

The match in Gelsenkirchen was a tense affair between the two rivals, especially as Dortmund had beaten Schalke 2-0 on the penultimate weekend of last season to deny the Royal Blues their first Bundesliga title.

"In the heat of the moment, things are often said which people later regret, and tensions were high for that game," Ghana-born Asamoah said.

"But when you hear something like that, it makes you really disappointed.

"He regretted saying it and he apologised to me during the game, but I don't know why he didn't admit it after the match.

"He should tell the truth."

In September 2006, monkey cries made by supporters of Hansa Rostock led to the DFB fining the club US$26,000.