WASHINGTON - US President Barack Obama on Thursday apologized for a counterterrorism operation in January that accidentally killed two aid workers held hostage by al-Qaida, American Warren Weinstein and Italian Giovanni Lo Porto. 'As a husband and as a father, I cannot begin to imagine the anguish that the Weinstein and Lo Porto families are enduring today,' Obama told reporters, with a deep sigh, saying he took responsibility for the deaths and has ordered a full review. 'I profoundly regret what happened,' Obama said, explaining he declassified some of the details of the operation so that the families could know what happened.