As yet, the government has not announced a new policy.

as 在这里是连词，表示"在...的时候"或"随着"。 yet 是副词，表示"还、尚、到目前为止"。 as yet 是一个固定搭配，意思是"到目前为止、迄今"，强调从过去到现在这段时间内某事尚未发生。