A suicide car bomber attacked the heavily fortified Afghan capital Kabul early Tuesday, killing at least three people, police said.

Initial reports of the explosion in the western part of the city indicated that American vehicles were targeted, said Abdul Ghafor Sayedzada, the chief of the city police's criminal investigation unit. The area around the blast site is also home to Afghan government buildings.

An Associated Press reporter at the scene saw the wreckage of a public bus and and four sport utility vehicles. The SUVs were painted white and grey but no markings identified them as American were immediately discernable.

U.S. soldiers and Afghan police were working to secure the site as news trickled out of deaths and injuries. A police officer at the scene, Habibullah Mohammadi, said he saw three dead bodies and an AP reporter saw one dead body carried away by police.

At least 12 wounded people were evacuated to hospitals, said Mirza Mohammad, a doctor who was treating the injured.

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 defences are still permeable by determined attackers.