German President Horst Koehler on Monday surprisingly announced his resignation following criticism of his controversial remarks on Germany's military deployment abroad.

"I declare my resignation from the office of President with immediate effect," said Koehler in Berlin, adding that he regrets that his remarks concerning German military deployment abroad in a recent radio interview has led to misunderstandings.

Koehler stressed meanwhile that claims that he "advocates an unconstitutional use of the German armed forces" are not justifiable.

Koehler said he has informed Jens Boehrnsen, speaker of the upper house of the German parliament, of his resignation. Boehrnsen will temporarily assume the presidential duty until a successor is chosen.

Koehler has triggered public criticism with a statement in a May 22 interview with Germany's Deutschlandradio that German military involvement is also necessary to protect the country's economic interests.

He was quoted as saying in the interview that a country like Germany with a heavy reliance on foreign trade "must also understand that in certain cases, in an emergency, military operations are necessary to protect our interests." He cited as examples maintaining free trade routes and preventing regional instability, which could have a negative impact on Germany's trade, jobs and income.

Koehler, however, stressed later that he had been misunderstood and that his remarks did not refer to the Afghanistan mission but rather to occasions such as military missions against pirates.

Although Koehler's role as head of state is largely ceremonial, his resignation is seen as a blow to Chancellor Angela Merkel, an important conservative political ally.

Merkel told a press conference in Berlin that Koehler called her Monday morning to inform her of his decision and she had tried in vain to dissuade him from resigning.

Merkel said she "deeply regrets" Koehler's decision to step down and sees him as an important adviser at times of economic and financial crisis.

Koehler is the first president to resign in Germany's postwar history. Koehler, 67, has held the post since July 2004 after reelection last year.