Page "Consonant" Paragraph 10

Other languages use fricative and often trilled segments as syllabic nuclei, as in Czech and several languages in Democratic Republic of the Congo, and China, including Mandarin Chinese.

In Mandarin, they are historically allophones of, and spelled that way in Pinyin.

Ladefoged and Maddieson call these " fricative vowels " and say that " they can usually be thought of as syllabic fricatives that are allophones of vowels ".

That is, phonetically they are consonants, but phonemically they behave as vowels.