German Foreign Minister Frank-Walter Steinmeier said Wednesday that he supports a "regional solution" that includes Pakistan in the fight against terrorism in Afghanistan as a suicide attack wounded five German soldiers in northern Afghanistan, according to dpa. "As long as this is not the case, our efforts in Afghanistan will only have limited success," Steinmeier said after a meeting with Afghan President Hamid Karzai. Pakistan's border region with Afghanistan serves as a refuge for militants who often launch attacks into Afghanistan. Steinmeier arrived Wednesday in Kabul on a surprise visit, where he was also expected to meet his Afghan counterpart, Rangin Dadfar Spantar. Steinmeier's trip, his fourth to Afghanistan since taking office at the end of 2007, was kept secret until after his arrival for security reasons. Five German soldiers were injured in a suicide attack in the northern Afghan province of Kunduz Wednesday. The bomber, who had strapped explosives around his body, carried out the attack in the Ali Abad district, provincial Governor Mohammed Omar said. Taliban spokesman Zabiullah Mujahid claimed responsibility for the attack but said the bomber had carried out the attack with an explosives-laden vehicle. "Two German military tanks were destroyed in the attack and all the German soldiers inside the tanks were killed," Mujahid said by phone from an undisclosed location. Germany has about 3,800 troops deployed to Afghanistan and plans to raise troop levels to 4,400 in the coming months as Afghanistan prepares for a presidential election in August. The minister offered German support in rebuilding Afghanistan, promising to help the country "get back on its feet after many years of war and civil war." Incumbent Karzai, who has been in office since 2001, has faced rising criticism over the past months from his Western backers, who accuse him of not doing enough to stem corruption in the country. British Prime Minister Gordon Brown on Monday called the Afghan-Pakistani border region a "crucible of terrorism," also with the view of the rising influence of Taliban militants in Pakistan. Karzai and Pakistani President Asif Ali Zardari are to meet US President Barack Obama in Washington next week.