WASHINGTON (Reuters) - The United States cannot permit northern Mali to become a safe-haven for Islamist rebels that could eventually pose a more direct threat to U.S. interests, Secretary of State Hillary Clinton said on Wednesday. Clinton, testifying to a Senate committee about the deadly September attack on the U.S. diplomatic mission in Benghazi, Libya, called the growing international campaign against Islamist fighters in northern Mali a response to "a very serious, ongoing threat." "We are in for a struggle, but it is a necessary struggle. ...