The earthquake left more than 25,000 dead in both countries, according to the latest balances, and damaged dozens of hospitals.
The WHO, which has already released $16 million from its emergency fund, said on Tuesday that up to 23 million people could be affected.
But the organization now puts the number at 26 million, with 15 million in Turkey and about 11 million in war-torn Syria.
Of this number, more than five million people are considered particularly vulnerable, including 350,000 elderly people and more than 1.4 million children.
The WHO estimates that in Turkey, where more than 4,000 buildings collapsed, 15 hospitals suffered partial or serious damage.
