KABUL, Afghanistan – A powerful car bomb detonated outside the office of an American security contractor in the Afghan capital yesterday, killing at least seven people, including two Americans, and wounding several others, officials and witnesses said.

Hours earlier, a blast wrecked a religious school in southeastern Afghanistan, reportedly killing at least eight children and one adult and underlining the country’s fragile security as it moves toward its first post-Taliban election in October.

Security officials have issued several warnings in recent weeks about possible car bombings and suicide attacks in the Afghan capital.

NATO forces patrolling Kabul have warned that anti-government terrorists, including the ousted Taliban, could try to mount spectacular attacks in a bid to disrupt the landmark presidential election scheduled for October 9.

The Kabul explosion hit the office of Dyncorp Inc., an American firm that provides security for President Karzai and works for the American government in Iraq, said Nick Downie of the Afghanistan NGO Security Office.

“The explosion…killed at least seven people,” Mr. Karzai’s office said in a statement. “Two Americans, three Nepalese and two Afghan nationals, including a child, have been confirmed dead.” Mr. Karzai and the American ambassador, Zalmay Khalilzad, expressed shock at the bombing.

An American Embassy statement said the contractor also was involved in a project to train Afghan police, a key element of the internationally backed plan to prevent the country from reverting to a haven for Al Qaeda. The company is believed to employ Nepalese and Americans in Afghanistan, where it reportedly is involved in anti-drug efforts.

Dyncorp Inc. is a division of Computer Sciences Corp. based in El Segundo, Calif. The blast occurred in Kabul’s Shar-e Naw district, a bustling area with the offices of international organizations and guesthouses used by their staff.