The number of people with Ebola in west Africa has risen above 16,000, with the death toll from the outbreak reaching almost 7,000, the World Health Organisation (WHO) says.

The number of deaths is more than 1,000 higher than the figure issued by the WHO just two days ago, but it is thought to include deaths that have gone unreported in the weeks or months since the outbreak began. Most of the new deaths were recorded in Liberia.

The WHO has warned that its figures could be a significant underestimation of the number of infections and deaths. Data from the outbreak has been patchy and the totals often rise considerably when backlogs of information are cleared. The latest confirmed data shows that almost half those known to have been infected with Ebola have died.

Meanwhile, two children tested for Ebola after arriving in Britain from Africa are not infected, Public Health England confirmed on Saturday. It said the overall risk to the public of the virus continued to be “very low”.

The children, whose ages and names have not been released, underwent precautionary tests in Newcastle for both the virus and malaria.

The outbreak has been centred on Guinea, Liberia and Sierra Leone. They account for the vast majority of the cases reported to date, with about three dozen cases elsewhere.

Liberia has recorded the highest number of cases and deaths, but the rate of infection is slowing there. The disease is now spreading fastest in Sierra Leone.

Mali has started recording infections after sick people crossed over from neighbouring Guinea. It has reported two new cases this week.

This outbreak has been the worst partly because it occurred in a highly mobile region, where Liberia, Guinea and Sierra Leone meet, and quickly spread to their respective capital cities.

Another UN agency, the Food and Agriculture Organisation, warned that families in the three countries were at risk of both malnutrition and under-nutrition.

Vincent Martin, of the FAO, said 70% of people interviewed in Sierra Leone had been eating only one meal a day since the outbreak, rather than two or three. Restrictions on movement had led to panic buying, food shortages and severe price hikes, the agency said.

The WHO said this week that the Ebola outbreak in the Democratic Republic of Congo had ended, as it did in Nigeria in late October.

Its guidelines state that a country can be declared free of the virus once 42 days have passed and no new cases have been detected. The 42 days represents twice the maximum incubation period for Ebola.

Scientists said on Thursday that progress towards creating an Ebola vaccine had been made. An experimental vaccine has triggered promising immune responses from 20 healthy volunteers in a preliminary trial, suggesting that it should protect against infection.

Trials of a device that can diagnose an Ebola infection within 15 minutes are about to start in Guinea. The test, which can analyse blood or saliva, is six times faster than those being used in west Africa.