At the same time, General Allyn says the military units must continue to transport food, water and other critical items that are needed immediately.

VOA: standard.2010.01.19

The article "an" or "a", you use when you're talking about like either a general category of items or

冠词中，“an”或“a”，是当你谈论那些一般的或者可选择的条目时用的，

经常犯的失误 - SpeakingMax英语口语达人