German President Horst Koehler resigned Monday in a surprise move after being criticized for reportedly linking military deployments abroad with the country's economic interests _ creating a new headache for Chancellor Angela Merkel.

The resignation, effective immediately, came only a year into Koehler's second term as the largely ceremonial head of state. Merkel had installed the former International Monetary Fund boss as president in 2004.

The current president of parliament's upper house _ Bremen Mayor Jens Boehrnsen, a member of the opposition Social Democrats _ will take over presidential duties temporarily, largely signing legislation into law.

A new president must be elected within 30 days. German politicians now will have to figure out quickly who should replace Koehler even as they are preoccupied with trying to make budget cuts amid the eurozone debt crisis.

Merkel called off a planned visit Monday to the German World Cup team's training camp in Italy.

Koehler, a member of Merkel's Christian Democrats, cited a week of criticism over a radio interview he gave following a visit to German troops in Afghanistan.

He said in that broadcast that, for a country with Germany's dependency on exports, military deployments could be "necessary ... in order to defend our interests, for example free trade routes."

That was taken by many as relating to Germany's unpopular mission in Afghanistan, although his office later said that he was referring to anti-piracy patrols off the coast of Somalia. Germans are often uneasy about deployments abroad, given the country's militaristic past, and the mission in Afghanistan makes many particularly uncomfortable.

Opposition politicians had called for Koehler to take back the remarks and accused him of damaging public acceptance of German military missions abroad.

"I regret that my comments in an important and difficult question for our nation were able to lead to misunderstandings," a strained-looking Koehler told reporters at the president's Bellevue palace.

Koehler complained that some critics had suggested he supported "Bundeswehr missions that are not covered by the constitution."

"This criticism lacks any basis," Koehler said in a statement delivered alongside his wife, Eva Luise. "It also is lacking in the necessary respect for the presidential office."

He added "it was an honor to serve Germany as federal president" then walked off without taking questions.

Koehler positioned himself as an outsider to Germany's political elite and enjoyed high popularity ratings. He occasionally refused to sign bills into law due to constitutional concerns, and once warned politicians against using the global financial crisis as a "backdrop for posturing."

The controversy over his military remarks wasn't particularly major and appeared to be petering out. Still, it added to a general sense of government drift.

The presidency is supposed to be above the political fray and carries little real power but traditionally functions as the nation's moral voice. The president is chosen by a special assembly of lower-house lawmakers and representatives of Germany's 16 states.

____

Associated Press writer Kirsten Grieshaber contributed to this report.