"Some of these giraffe populations number only a few hundred individuals and need immediate protection."
The most threatened potential species include the reticulated giraffe (currently Giraffa camelopardalis reticulate) in Kenya, Ethiopia and Somalia.
Its population was estimated at around 27,000 until the 1990s, when poaching and conflicts slashed its numbers to just 3,000.
In west and central Africa, there are thought to be only 160 Nigerian giraffes (Giraffa camelopardalis peralta) left.
But all giraffes were under threat, Brown said in a statement, citing an estimated 30 per cent drop in numbers over the past decade.
Classifying what are currently seen as sub-species as fully-fledged species would force governments and experts to re-examine conservation efforts, he said.
Giraffes are the world's tallest land animal and can grow up to 5.8 metres tall and weigh 2,000kg.