The United States, concerned that Iran is behind a string of cyberattacks against U.S. banking sites, has considered delivering a formal warning through diplomatic channels but has not pursued the idea out of fears that doing so could escalate hostilities, according to American officials. It also reflects the pressure the administration is under from banking industry officials, who want to know what amount of pain or damage will justify a government response. “We don’t have a clear view of what are the triggers — and we’ve asked,” said one industry official who has been involved in discussions with the administration and who spoke on the condition of anonymity. Administration officials say it is difficult and unwise to be too precise about potential responses because they do not want to set red lines that, if crossed, might obligate them to act.