Mental illnesses are the world's leading cause of disability affecting millions and, even during a humanitarian crisis, treating them is not an optional luxury, experts said before a World Bank/World Health Organization meeting on the issue in Washington this week.

The number of people affected by humanitarian crises has nearly doubled in the past decade, and the number of people forced to flee their homes - more than 60 million - is at its highest since World War Two.

The WHO estimates the prevalence of common mental disorders, including anxiety and depression, can double during a humanitarian crisis to up to 20 percent of the population from 10 percent in normal times.

"The number of people affected by humanitarian crises is increasing globally, which also increases mental health needs," Inka Weissbecker, global mental health and psychosocial adviser at aid agency International Medical Corps, told the Thomson Reuters Foundation in an interview.

She is on one of the panels organized by the World Bank and WHO around the high-level meeting.

Some countries affected by war - like Sierra Leone and South Sudan - have only one or two psychiatrists to treat the entire population, she said.

Until recently, donors for South Sudan considered mental health care a luxury in a country with one of the world's highest maternal death rates, Weissbecker said.

"The problem with mental health is it's often invisible because it's not measured, so people such as donors or local governments may not think about it, or think it's important," she added.

Stigma and lack of knowledge are big barriers in all countries, with many people unaware that mental illnesses are treatable, Weissbecker said.

Mark van Ommeren, public mental health adviser at WHO, said: "We have a lot of data showing that mental health is a much bigger problem, a much bigger part of life, than people previously realized - it's one of the elephants in the room."

Wars and natural disasters can open a window for a fresh focus on mental health care, which in turn can be vital to improving a country's economy and overall development, he said.

A WHO-led study published on Tuesday said that every $1 invested in scaling up treatment for depression and anxiety led to a return of $4 in better health and ability to work.

The study estimates for the first time both the health and the economic benefits of investing in treatment, and is based on studies of 36 low, middle and high-income countries.

Many countries, including Afghanistan and Sri Lanka, have transformed their mental health care as a result of an emergency.

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Before the 2004 South Asian tsunami hit Sri Lanka, most of the island nation's mental health care was offered by hospitals in big cities, and the majority of people needing treatment did not receive it, according to a paper van Ommeren co-authored.

After the tsunami, the government realized more widespread care was needed to help the acute distress of survivors. It introduced community-based and comprehensive care, and set up a one-year diploma course to train more carers in mental health.

Afghanistan too has prioritized mental health since the fall of the Taliban government in 2001, and it aims to roll out access to some form of care in all primary health clinics across the country.

Other countries that have improved their mental health care include Burundi, Indonesia's Aceh province, Iraq, Jordan, Kosovo, East Timor, Lebanon and now Syria.

"The interest in mental health in and after an emergency is enormous.

"It's not surprising to see that the countries who have made the greatest progress in mental health system development are those that have been through emergencies," van Ommeren said.

Although aid agencies are offering much more mental health care in emergencies than just a few years ago, the gap between what is in place and what is needed remains huge, he said.

"We are able to do very good things for people that are helped - but most people are not helped," he said.