By Joseph Ax, Nicholas Brown and Sarah Marsh NEW YORK/BUENOS AIRES (Reuters) - Argentina cannot turn its back on negotiations with holdout creditors after defaulting on its sovereign debt, a U.S. judge instructed on Friday, just as the country's failure to service a June interest payment was declared a "credit event." In a stern tone, U.S. District Judge Thomas Griesa in New York slammed the decision by Latin America's third-biggest economy to defy his order to pay holdout investors in full and instead default on $29 billion in debt. As Griesa was speaking, a 15-member committee facilitated by the International Swaps and Derivatives Association (ISDA) voted unanimously to call the missed coupon payment a "credit event." The move triggers a payout process for holders of insurance on Argentine debt, which analysts estimate could amount to roughly $1 billion. Argentina's economy ministry said later in a combative statement that Griesa's attitude sought to favor "vulture funds". It has asked Argentina's securities watchdog to investigate whether the litigation against the nation by holdouts was merely the "facade of speculative maneuver".