Apple filed suit against Samsung last week because it had "crossed the line," a top Apple executive said Wednesday, but hopes to maintain a "strong relationship" with the South Korean company. "We are Samsung's largest customer (for liquid crystal display panels and semiconductors) and Samsung is a very valued component supplier to us," Apple chief operating officer Tim Cook said. "And I expect the strong relationship will continue," Cook told financial analysts during a second-quarter earnings call. "Separately from this, we felt the mobile communications division at Samsung had crossed the line and after trying for some time to work the issue we decided we needed to rely on the courts," Cook said. Apple filed suit against Samsung in San Francisco on Friday alleging that the South Korean giant copied its smartphones and tablet computers. Apple's lawsuit claims Samsung's mobile phones and Galaxy Tab imitated the iPhone and the iPad. Samsung vowed it would "respond actively to this legal action taken against us through appropriate legal measures to protect our intellectual property." Apple was Samsung's second-largest client in 2010 after Japan's Sony Corp, accounting for four percent of the South Korean firm's 155 trillion won ($142 billion) annual revenue. Samsung's Galaxy Tab has been the best-selling rival to the iPad, which has dominated the growing market for the touchscreen devices.