Nearly one billion people, including around one in seven teenagers, worldwide suffer from some form of mental disorder, according to the UN.

To make matters worse, in the first year of the Covid-19 pandemic, rates of common conditions such as depression and anxiety went up by more than 25 percent, the UN health agency said Friday.

In its largest review of mental health since the turn of the century, the World Health Organization urged more countries to get to grips with worsening conditions.

It offered examples of good practices that should be implemented as quickly as possible, in recognition of the important role that mental health plays in positive and sustainable development, at all levels.

Even before Covid hit, only a small fraction of people in need of help had access to effective, affordable and quality mental health treatment, the WHO said, citing the latest available global data from 2019.

More than 70 percent of those suffering from psychosis worldwide do not get the help they need, the UN agency said.

Read: Mental Health: Types of Mental Illness and supporting someone with a mental health problem

The gap between rich and poor nations highlights unequal access to healthcare, as seven in 10 people with psychosis receive treatment in high-income countries, compared to only 12 percent in low-income countries.

The situation is more dramatic for cases of depression, the WHO said, pointing to gaps in assistance across all countries – including high-income ones – where only one-third of people who suffer from depression receive formal mental health care.

And although high-income countries offer "minimally adequate" treatment for depression in 23 percent of cases, this drops to just three percent in low and lower-middle-income countries.