Aug 17 (Reuters) - NATO only had the option to leave Afghanistan completely or to increase troop numbers and resume combat operations there, NATO Secretary-General Jens Stoltenberg said on Tuesday. "Allies took that risk (of the withdrawal) clear-eyed because they knew that the alternative was not to continue with a limited military presence, the alternative was most likely to continue with an increased presence of NATO troops and to once again engage in combat," he told reporters in Brussels. "The frustration is easy to understand when we see that so many years of efforts by the whole international community have not given better and stronger results when it comes to the Afghan state structures."