Aid agencies say it is critical that medical and food supplies reach some 300,000 people displaced in Afghanistan over the past two months amid advances by the Islamist Taliban insurgents that culminated in the capture of Kabul on Aug 15.
Nearly 18.5 million people - half the population - rely on aid and the humanitarian needs are expected to grow due to drought.
"While the eyes of the world now are on the people being evacuated and the planes leaving, we need to get supplies in to help those who are left behind," WHO spokesperson Inas Hamam said in an emailed statement to Reuters.
She said the WHO was calling for empty planes to divert to its warehouse in Dubai, United Arab Emirates, to collect the supplies on their way to pick up evacuees from the country.
The WHO is also looking at setting up a "humanitarian air bridge" to send in supplies, she said.
The executive director of the UN children's agency UNICEF, Henrietta Fore, said on Monday around 10 million children across Afghanistan need humanitarian assistance and that conditions are expected to deteriorate further.