KHOST, Afghanistan - At least 18 civilians, including two police officers, were killed on Wednesday by a suicide bomber who attacked an American and Afghan military convoy in this provincial capital, Afghan officials said. Witnesses spoke of American and Afghan military casualties. A spokesman for the NATO-led International Security Assistance Force, Maj. Martyn Crighton, said the attack had "caused some ISAF casualties," but that it was too soon to say how many. Major Crighton said the military was aware of reports of a large number of military casualties but did not yet have details on how many were Afghan as opposed to coalition soldiers, or how many were wounded or killed. He confirmed that ISAF and Afghan soldiers were present at the scene of the attack. Officials at two Afghan hospitals said a total of 18 civilians were brought to those facilities, including two policemen and some children. NATO and Afghan military casualties would normally be evacuated to military facilities. Malik Noor, a doctor in Babrigul Hospital, which received many of the victims, said seven dead bodies were brought there, including those of two policemen and some children. At the Khost Regional Hospital, the dead totaled 11, including two children, plus more than 16 wounded, according to Dr. Majid Mangal, the hospital director. The attack was carried out by a suicide bomber on foot near the Spin Jomat bazaar in the crowded downtown area of Khost, capital of the eastern province of the same name, said Sardar Mohammad Zazai, the Khost Province Police Chief. He said the bomber detonated an explosive vest as the convoy was passing through about noon. However, the size of the blast and the number of casualties was unusually large for a lone bomber, and the authorities were still investigating the circumstances. Mr. Zazai said he had no official tally of casualties. A man whose shop is close to the blast site said: "I was busy in my shop when I heard a deafening explosion and then a wave of heat, flying sand and dust hit me in the face. One month ago, on May 19, a suicide bomber in Khost attacked a police post, killing three policemen and at least two children.