(Adds AU comment, details and share prices, updates dateline)
* Flat-screen patents at center of dispute
* Lawsuit follows Samsung complaint against AU
* AU also sues AT&T, Best Buy and Brandsmart
TAIPEI/WILMINGTON, June 29 (Reuters) - AU Optronics Corp of Taiwan hit Korea's Samsung Electronics Co Ltd with a countersuit for patent infringement, ratcheting up their dispute over flat-panel technology.
AU Optronics also sued AT&T , Best Buy Co Inc and BrandsMart U.S.A. Inc, accusing them of selling or distributing infringing technology used in televisions, monitors, laptops, mobile phones and tablet computers.
"We need to take action in order to safeguard the interest of the company and our clients," said AU spokeswoman Freda Lee.
She added that AU has made an additional three charges in the filing, including that Samsung has infringed its patented technologies on OLED devices used in mobile phones.
The lawsuit, filed on Monday in Delaware District Court, seeks an order to stop the alleged infringement as well as monetary damages. AU Optronics, the world's No.4 maker of flat screens, asked the court to treble any monetary award because the infringement was willful.
By 0125 GMT, AU shares were up 1.3 percent, while Samsung rose 2 percent, both outperforming their broader markets .
Earlier this month, Samsung filed a complaint against AU Optronics with the International Trade Commission and in federal court for patent infringement related to flat screens.
"Samsung Electronics is committed to respecting the intellectual property rights of others," the Korean company said in a statement. "However, we will also work actively to protect our rights and our customers when another party makes unsubstantiated infringement claims against Samsung."
AU clients included in the suits are computer maker Acer Inc , Sanyo Electric (Taiwan) Co Ltd, , and electronics firm BenQ Corp, a Samsung spokesman said earlier.
Patent disputes are common in the technology sector, especially for flat screens and semiconductors, as makers seek to protect their newest technologies from being commoditised and exploited by rivals. But most are settled out of court as big companies prefer to avoid long fights and patented technology can be out of date by the time a case is over.
Samsung vies for the top flat-screen position with Korean rival LG Display Co Ltd .
AT&T spokesman Mark Siegel said the company had not reviewed the lawsuit and declined to comment. Best Buy declined to comment and BrandsMart did not reply to a request for comment.
The case is AU Optronics Corp v Samsung Electronics Co Ltd, U.S. District Court, District of Delaware, No. 11-568. (Reporting by Tom Hals in Wilmington and Clare Jim in Taipei; Editing by John Wallace, Robert MacMillan and Matt Driskill)