Mental illness affects one-in-five people living in war zones, according to the World Health Organization.
The WHO's conflict mental health study, published in The Lancet medical journal, found many suffered severe forms of depression, anxiety, post-traumatic stress, bipolar disorder or schizophrenia.
The findings highlight the long-term impact of war-induced crises in countries such as Afghanistan, Iraq, South Sudan, Syria and Yemen, the UN's health agency said.
The study also found that rates of depression and anxiety in conflict settings appeared to increase with age, and depression was more common among women than men.
12% of the world's people are living in an active war zone, according to United Nations figures.
"Given the large numbers of people in need and the humanitarian imperative to reduce suffering, there is an urgent need to implement scalable mental health interventions to address this burden," the research team said.
Mark van Ommeren, a mental health specialist at the WHO who worked on the team, said the findings "add yet more weight to the argument for immediate and sustained investment, so that mental and psychosocial support is made available to all people in need living through conflict and its aftermath".
In 2016, the number of ongoing armed conflicts reached an all-time high of 53 in 37 countries.
Since World War Two, almost 69 million people globally have been forced to flee war and violence.
The WHO's study was carried out by a team of researchers from the WHO, Australia's Queensland University, and the Institute for Health Metrics and Evaluation at the University of Washington and Harvard University in the United States.
It analysed research from 129 studies and data from 39 countries published between 1980 and August 2017.
Regions that have seen conflict in the last ten years were included - natural disasters and public health emergencies, such as Ebola, were not.
The study found that rates of depression and anxiety in conflict settings appeared to increase with age, and depression was more common among women than men.
The study was funded by the WHO, the Queensland Department of Health and the Bill & Melinda Gates Foundation.