"Such leaks are very unfortunate and may have very negative security implications for people concerned," Rasmussen said during a joint press conference with German Chancellor Angela Merkel in Berlin.
"Leaks may put soldiers as well as civilians at risk," he added.
WikiLeaks announced Monday it would be releasing more documents very soon, but did not specify exactly what was coming.
The US Pentagon said last week it was scouring an Iraq war database to prepare for potential fallout from an expected release by WikiLeaks of some 400,000 secret military reports.
It appealed to the media to avoid facilitating the leak of documents regarding Iraq.
But in a tweet on the micro-blogging networking site Twitter, Wikileaks said: "We did not say we were publishing something on Iraq."
The whistleblower website published 77,000 classified US military documents on the war in Afghanistan in July.
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