Afghanistan Suicide Blast Kills 5 U.S. Troops

Updated at 8:20 a.m. Eastern.

Six NATO service members, including five Americans, and at least 12 civilians were killed Tuesday in a suicide car bombing in the Afghan capital.

A U.S. military spokesman in Afghanistan said five Americans were among the six NATO troops killed by the bomber in the Afghan capital today.

Afghan officials said earlier the bombing also killed at least 12 Afghan civilians and injured 47.

The bomber struck during rush hour on a major road running through Kabul. Nearly 20 vehicles were damaged, including a bus on which most of the civilian casualties were riding.

A Taliban commander in Kabul city, Qari Talah, claimed responsibility for the attack in a phone call to CBS News' Sami Yousafzai.

A bomber "hit a convoy of the ISAF forces this morning at local time 8:20 a.m. and killed a number of foreigners," Talah claimed to CBS News.

An Associated Press reporter on the scene saw the wreckage of a public bus and four sport utility vehicles. The SUVs were painted white and grey but no markings identifying them more specifically were discernable.

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Interior Ministry spokesman Zemeri Bashary said the blast was near the Afghan Ministry of Energy and Water.

A spate of attacks inside Kabul this year has led police to tighten security and officials have recently publicized arrests of would-be suicide bombers as proof that they are having success. The Tuesday bombing is a reminder that the city's defenses are still permeable by determined attackers.

The U.S.-led force in Afghanistan is bracing to push hard into the Taliban's birthplace in Kandahar Province in June. The campaign for Kandahar, already under way in districts outside the city, is expected to be among the bloodiest of the nearly nine-year-old war.

It may well be the most critical battle of the war, reports CBS News correspondent David Martin.

More than 20,000 U.S. and allied troops are gearing up for the make-or-break operation to retake the city of Kandahar from the Taliban. According to Stephen Biddle, a civilian adviser to Gen. Stanley McChrystal, the outcome of the entire war is riding on it.

"If we fail to secure this population, it's hard to see how the campaign could succeed," Biddle told CBS.

"Our summer offensive has started and a major part of the offensive will bring more death to the enemies inside cities," warned the Taliban commander Talah in his phone call to CBS. "If U.S. forces go after Taliban in Kandahar, we will go after each convoy of U.S., NATO and ISAF forces in Kabul."

"Soon, each convoy will be escorted by bombers all over Afghanistan," Talah added.