An explosion shook the headquarters of NATO's 8,000-strong security force in the Afghan capital on Monday, but there were no immediate reports of injuries, a spokeswoman for the force said. The blast occurred "in the vicinity" of the International Security Assistance Force compound, said Lt. Col. Karen Tissot Van Patot. She said officers were investigating the cause of the explosion. An Afghan police officer outside the compound, speaking on condition of anonymity, said a rocket had hit inside the heavily fortified base, which is near the U.S. Embassy and other diplomatic missions in central Kabul. Sirens wailed across the city for about 30 minutes after the blast.