Mali authorities on Saturday sought to calm fears after Ebola claimed its first victim in the African country, a contagious toddler who took a 1,000-kilometre bus journey before being treated.

The World Health Organization (WHO) warned the situation in Mali was an "emergency", and said in its latest Ebola situation report that the biggest outbreak on record has now killed 4,922 people, the vast majority of them in Guinea, Liberia and Sierra Leone, with 10,141 cases reported.

The US states of New York and New Jersey ordered mandatory quarantine for medics who had treated victims of the disease in west Africa, after a doctor who had returned from the region became the first Ebola case in New York City.

President Barack Obama told Americans on Saturday that they must be "guided by the facts, not fear". He sought to calm a jittery public by hugging one of the two nurses who became the first to contract Ebola on American soil after treating a patient, but has now been declared free of the disease.

Mali President Ibrahim Boubacar Keita aimed to ease fears after the death of a two-year-old girl, the first Ebola case in the landlocked country, who travelled from neighbouring Guinea.

"We are doing everything to prevent panic," he said in an interview with French radio.

"Since the start of this epidemic, we in Mali took all measures to be safe, but we can never hermetically seal ourselves from this," he said.

"Guinea is a neighbouring country, we have a common border that we have not closed and that we will not close."

Mauritania meanwhile reinforced controls on its border with Mali, which led to a de facto closing of the border, according to local sources.

Mali 'emergency'

The WHO said it was treating the situation in Mali as an "emergency" because the toddler had travelled for hundreds of kilometres on public transport with her grandmother while showing symptoms of the disease -- meaning that she was contagious.

"The child's symptomatic state during the bus journey is especially concerning, as it presented multiple opportunities for exposures," the UN agency said.

The girl and her grandmother travelled by public transport from Keweni in Guinea through the towns of Kankan, Sigouri and Kouremale to the Malian capital Bamako.

"The two stayed in Bamako for two hours before travelling on to Kayes," in Mali's southwest, where treatment was sought for the child, the WHO said.

The route made for a journey of around 1,000 kilometres (600 miles).

"Bleeding from the nose began while both were still in Guinea, meaning that the child was symptomatic during their travels through Mali."

Mali's health ministry however denied that the girl had been showing symptoms before she reached Kayes.

The Malian authorities were tracing everyone who had contact with the girl and her grandmother and placed more than 50 people under observation.

One metric tonne of medical supplies was dispatched from WHO stocks in Liberia to Bamako late Friday.