LONDON — About 11 percent of medicines in developing countries are counterfeit and likely responsible for the deaths of tens of thousands of children from diseases like malaria and pneumonia every year, the World Health Organization said Tuesday.

It’s the first attempt by the U.N. health agency to assess the problem. Experts reviewed 100 studies involving more than 48,000 medicines. Drugs for treating malaria and bacterial infections accounted for nearly 65 percent of fake medicines.

WHO’s director-general said the problem mostly affects poor countries. Between 72,000 and 169,000 children may be dying from pneumonia every year after receiving bad drugs. Counterfeit medications might be responsible for an additional 116,000 deaths from malaria mostly in sub-Saharan Africa, according to scientists at the University of Edinburgh and London School of Hygiene and Tropical Medicine commissioned by the WHO.

Counterfeit drugs include products that have not been approved by regulators, fail to meet quality standards or deliberately misrepresent an ingredient, according to WHO, which published the reports.

In 2013, WHO set up a voluntary global monitoring system for substandard and fake drugs and has received reports of about 1,500 problematic medicines.