Borussia Dortmund CEO Hans-Joachim Watzke has hit out at Bayern Munich counterpart Karl-Heinz Rummenigge following his latest remarks on Marco Reus.
Watzke had already questioned why Rummenigge was speaking about his club when the Bayern CEO told the Welt am Sonntag newspaper last month that Dortmund might find it "difficult" to remove Reus' release clause from his contract.
This week, Rummenigge again hinted at his club's interest in the Germany international and told Sport Bild: "It's common knowledge that he is a Borussia Dortmund player and that he has a release clause, which, rumour has it, is worth 25 million euros."
He also told ESPN FC ahead of Wednesday's game against MLS All-Stars that Reus is a "very interesting player" and that his release clause is "dangerous" for BVB as 25 million euros is "not a high price."
Rummenigge, though, said he had "a very respectful relationship" with Watzke and added: "I don't want to strain our relationship with Borussia Dortmund."
However, in a statement published on BVB's official website, Watzke said: "Borussia Dortmund have registered with a certain degree of annoyance that Karl-Heinz Rummenigge is currently saying something about Borussia Dortmund matters at a rate of once every three days.
"We are sure that it is quite clear to anybody interested in football what intentions lie behind such remarks.
"Karl-Heinz Rummenigge is also fully aware that his behaviour is only going to further damage an already tense relationship between Borussia Dortmund and Bayern Munich.
"Naturally as employers, we are going to continue to follow our principles in future and not discuss any details of our employees' contracts in public."
Watzke's words are a response to the "repeated comments from FC Bayern Munich over recent weeks about the contractual details of Borussia Dortmund's employees," according to the statement.
Bayern have already signed two key players from Dortmund in recent times, paying Mario Gotze's buyout clause last year before bringing in Robert Lewandowski on a Bosman transfer this summer.
There had been speculation that Bayern had agreed to stop poaching players from their rivals, but Rummenigge denied that was the case in the Welt am Sonntag interview.
Dortmund's director of sport Michael Zorc added to kicker.de: "It's moved onto an entirely new level. It would be nice if Karl-Heinz Rummenigge could keep his mouth shut once in a while."
The two clubs will renew their rivalry in just a week's time when they meet in the German Super Cup, the curtain-raiser to the Bundesliga season.