The U.S. military in Iraq says a joint mission by American and Iraqi troops has killed or arrested six suspected al-Qaida leaders accused of running an extortion and assassination ring in the north of the country. A U.S. military statement said the operation was carried out from March 18-24 in Mosul, 225 miles (360 kilometers) northwest of Baghdad. It says the network helped fund al-Qaida in northern Iraq by targeting oil companies and small businesses for extortion.