Herat, Afghanistan ? A car loaded with explosives blew up next to a bus carrying German peacekeepers Saturday in Kabul, killing four soldiers in the first fatal attack on the international security force that has patrolled the capital for 18 months, a spokesman for the peacekeepers said.

The apparent suicide bombing — which would be the first such attack on the International Security Assistance Force — injured 31 soldiers, said the spokesman, Dutch Lt. Col. Paul Kolken.

The peacekeepers, some of whom had just finished their tour of duty, were on their way to the Kabul airport to catch a flight home to Germany when an automobile believed to be a taxi pulled alongside their convoy and exploded, Kolken said.

“They were overtaken by a taxi on the right-hand side of the bus,” Kolken said. “When the taxi was alongside the bus, it exploded. As far as we understand, there was one person in the taxi.”

No civilians were injured in the blast, which occurred in a business district in east Kabul, said Mohammad Rauf Taj, a police official.

No one claimed responsibility for the attack, Kolken said.

German Chancellor Gerhard Schroeder condemned the attack as the “cowardly and underhanded” work of terrorists, the Associated Press reported from Berlin.

Speaking to the German television network ZDF, German Defense Minister Peter Struck said his Afghan counterpart, Mohammed Fahim, told him there were indications al-Qaida may have been behind the attack. “I think that not only al-Qaida but former members of the Taliban … would try to chase the international security forces, including the Germans, out of Afghanistan,” Struck said.

Since the United States and its allies toppled the Taliban in late 2001, remnants of the Taliban, al-Qaida and other radical Islamic groups have vowed to drive foreign soldiers from Afghan soil and destabilize the U.S.-backed government of President Hamid Karzai.

U.S. and other foreign soldiers have been the frequent targets of attacks in recent months, but none of the 5,000-member peacekeeping force based in Kabul had been killed, though more than a dozen have died in accidents.

A German peacekeeper was killed and another wounded last month when their vehicle ran over a land mine, but officials said they believe the mine was left over from Afghanistan’s two decades of warfare.