WHO has documented 594 attacks resulting in 959 deaths and 1561 injuries in 19 countries with emergencies between January 2014 and December 2015.

Nearly 1000 people have been killed in attacks on health centres worldwide over the past two years, almost 40 per cent of them in Syria, the World Health Organisation (WHO) said in its first report on the growing scourge.

The United Nations agency documented 594 attacks resulting in 959 deaths and 1561 injuries in 19 countries with emergencies between January 2014 and December 2015.

Syria, torn by civil war since 2011, had the most attacks on hospitals, ambulances, patients and medical workers, accounting for 352 deaths. The Palestinian territories of Gaza and the West Bank, as well as Iraq, Pakistan and Libya, followed.

Some 62 per cent of all attacks were deemed intentional and many led to disruption of public health services.

"This is not an isolated issue, it is not limited to war zones, it is not accidental. The majority of these are intentional," Dr Bruce Aylward, executive director of WHO's emergency program, told a news briefing.

"It is also not stopping and it has real complications for what we are trying to do. It is getting more and more difficult to deploy people into these places, it is getting more and more difficult to keep them safe when they are there and it is getting more and more difficult to ensure they survive, let alone recover in crises."

The casualty figures include 42 killed and 37 wounded in a a US air strike on a Medecins Sans Frontieres (Doctors Without Borders) hospital in Kunduz, Afghanistan last October.

A US military report last month said that the incident did not amount to a war crime but was caused by human error, equipment failure and other factors, but MSF has called for an independent inquiry.

WHO said 53 per cent of the attacks were perpetrated by states, 30 by armed groups and 17 per cent remain unknown.

"One of the most important rules of war you is that you don't attack health care facilities, health care providers, the sick, the disabled. So these attacks do represent gross violations of international humanitarian law," said Rick Brennan, WHO director of emergency risk management and humanitarian response.

"Violations of international humanitarian law, if proved, can be considered war crimes and the perpetrators can be taken to the International Criminal Court," he said.