Five days before Afghanistan's presidential election, a huge explosion rocked the diplomatic district of Kabul, the capital, on Saturday morning. The blast, attributed to a suicide car bomber, struck just outside the headquarters of NATO's operation here, which is called the International Security Assistance Force and the Afghan Ministry of Transportation. 'We have casualties,' the Afghan Defense Minsitry spokesman, Gen. Zaher Azimi, said. 'We don't know how many.' A driver for the Transportation Ministry, Abdul Rahid, 35, said he was inside the ministry building at the time of the blast. He said that there were many casualties among people on the street and visitors to the building. The U.S. Embassy in Kabul is about 100 yards from the site of the explosion, but blast walls protect it from the street. The Afghan presidential palace is about 200 yards away. The Afghan president, Hamid Karzai, is seeking re-election on Wednesday. The Taliban have accelerated a campaign of violence to disrupt the voting. 