The United Nations has said that at least 450 civilians have likely been killed by U.S. drone strikes in Pakistan, Afghanistan and Yemen.

A U.N. report, obtained by Al Jazeera on Friday, stated that Pakistan’s government had confirmed more than 400 civilian deaths as a result of U.S. drone strikes and that at least 50 civilians were killed in strikes in Afghanistan and Yemen.

The U.S. provides little public information about drone deaths, but according to a New York Times exposé, the Obama administration considers “all military-age males (killed) in a strike zone” to be combatants “unless there is explicit intelligence posthumously proving them innocent.”

The classification is important. Critics argue that it undercounts the civilian death toll and that if a ‘militant’ killed, that implies the killing was lawful.

The report found that the major obstacle in obtaining accurate figures on civilian deaths was a lack of transparency by the countries involved. The U.S., according to the report, has an obligation to launch an impartial investigation and provide a public explanation because of its duty to protect civilians in armed conflicts.

So far, the U.S. has not released any casualty figures from its CIA-led drone program, but the spy agency says the totals were in single digits annually, according to media reports.

The report, to be presented to the U.N. General Assembly on Oct. 25, recommends that U.N. member states comply with international humanitarian law, and it identifies a number of legal questions on the use of drones that currently have no international consensus.