The number of people dying from malaria has almost halved since 2000, although progress in west Africa risks being reversed by the Ebola outbreak, the World Health Organization said Tuesday.

The UN agency also warned of major gaps in access to mosquito nets and anti-malaria treatments, as well as the worrying emergence of resistance to the most commonly used insecticides.

Worldwide, malaria deaths were down 47 percent between 2000 and 2013 and decreased 53 percent in children under the age of five, the WHO said in its annual report on the disease.

In sub-Saharan Africa, where 90 percent of all malaria deaths occur, the mortality rate decreased by 54 percent -- 58 percent in under fives, the equivalent of about 3.9 million children's deaths averted.

The number of infections in the region at any one time fell 26 percent during the same 13-year period.

Meanwhile 13 of the 97 malarial countries reported no cases of the disease last year, including two, Azerbaijan and Sri Lanka, which recorded their first ever zero result.

"These are truly unprecedented results and phenomenal news in terms of global health," said Pedro Alonso, director of the WHO's global malaria programme.

He attributed the progress in large part to increasing financial and political commitment, in particular regional efforts to work together to eliminate malaria.

However, despite a threefold increase in investment since 2005, malaria programmes are still underfunded -- $2.7 billion (2.2 billion euros) in 2013 against a $5.1 billion international target.

And as a result, major gaps remain.

While access to insecticide-treated bed nets has improved, 278 million of the 840 million people at risk in sub-Saharan Africa still live in households without one.

As many as 15 million pregnant women do not receive any preventative treatment, while 437,000 African children are still dying from the disease each year.

Ebola threatens gains

Any gains can be fragile. In Sierra Leone, Guinea and Liberia, the Ebola outbreak has halted malaria programmes in some areas and put huge strain on health services.

"The collapse of health systems has affected all core malaria interventions and is threatening to reverse recent gains," said WHO director-general Margaret Chan.

Health workers had been increasingly using simple tests to diagnose malaria on the spot, to better target treatments. But these have been suspended in Ebola areas.

Many people with malaria are also staying away from clinics, and if "they are not getting treated, you can be sure that mortality is going to increase", said Richard Cibulskis, lead author of the malaria report.

The total death toll from malaria across the three countries was expected to be about 20,000 a year before the outbreak. Cibulskis would not predict a figure now.

More than 6,100 people have died from Ebola in the region in the past year.

Aside from the direct consequences, the resurgence of malaria could also harm the fight against Ebola because the two have similar symptoms, making it difficult to diagnose the deadly virus, the WHO has said.

UNICEF last week launched a campaign to provide anti-malarial drugs to 2.4 million people in Sierra Leone, while global aid agency Doctors Without Borders is conducting a smaller scale effort in Liberia.

Another issue threatening progress on malaria across the world is the rise of insecticide resistance, which has been reported in 49 countries since 2010 -- 39 of which reported resistance to two or more insecticide classes.

"Emerging drug- and insecticide-resistance continues to pose a major threat, and if left unaddressed, could trigger an upsurge in deaths," Chan said.