A spokesman for peacekeeping soldiers patrolling the Afghan capital vowed Sunday that the international forces would "not step back a single step," a day after a homicide bombing killed four German soldiers and wounded 31 in the worst-ever attack on the security force.

"Let's make it absolutely clear that ISAF is here in Kabul because the situation is not yet stable and not yet 100 percent safe," said spokesman German Lt. Col. Thomas Lobbering of the resolve of the peacekeepers, called the International Security Assistance Force (search), or ISAF.

"This is also the reason why we will stand with you in the future and why we will continue to conduct our mission here although we know that in doing so, we expose ourselves to threats."

On Saturday, a man driving an explosives-laden taxi pulled up alongside a bus in eastern Kabul (searchcarrying 35 German peacekeepers, most headed to the airport after completing six-month tours of duty in this war-shattered country.

The taxi, packed with between 220 to 1,110 pounds of explosives, set off a blast. Lobbering said 35 Germans were on the bus, two more than previously announced. Four were killed and the rest injured, but Lobbering said the toll could have been worse.

"Due to the fact that they all wore their flak jackets, the number of people killed luckily is much less than one could expect otherwise," Lobbering said.

All of the 31 wounded Germans were expected to survive, he said, adding that 25 of them had been flown to Germany for medical treatment.

The driver of the taxi was also killed in the blast, but his nationality was not known. It was also not clear how many people were in the taxi, Lobbering said.

There has been no claim of responsibility for the blast, but suspicion immediately fell on remnants of Usama bin Laden's Al Qaeda (searchnetwork, supporters of the defeated Taliban regime and rebel leader Gulbuddin Hekmatyar. All three groups are nominal allies whom authorities blame for deteriorating security countrywide.

Lobbering declined to say what added security precautions the peacekeeping force would use in the future, but he said the use of buses would be temporarily suspended.

"It is a part of our task here in Kabul to take care of security, to take care together with the Afghan authorities of public order," Lobbering said. "We have to continue our task by (carrying out) foot patrols, by car patrols and that's exactly what we are doing ... So knowing about the threat we will not step back a single step."

The Germans killed were the first peacekeepers to die in Afghanistan as a result of hostilities since the 5,000-strong peacekeeping force was deployed in Kabul in December 2001 to help maintain law and order in the capital after the Taliban were ousted in a U.S.-led war.

Before Saturday's attack, 15 peacekeepers had died on duty in Afghanistan, all of them in accidents. Last month, 62 Spanish peacekeepers died when their plane crashed in Turkey as they were returning home.