By Ellen Nakashima , Major U.S. banks have turned to the National Security Agency for help protecting their computer systems after a barrage of assaults that have disrupted their Web sites, according to industry officials. The attacks on the sites, which started about a year ago but intensified in September, have grown increasingly sophisticated, officials said. The NSA, the world’s largest electronic spying agency, has been asked to provide technical assistance to help banks further assess their systems and to better understand the attackers’ tactics. The cooperation between the NSA and banks, industry officials say, underscores the government’s fears about the unprecedented assault against the financial sector and is part of a broader effort by the government to work with U.S. firms on cybersecurity. Nonetheless, the assistance is likely to dismay privacy advocates, who say that the NSA has no business peering inside private companies’ systems, even if for the strict purpose of impro