The first two cases of Ebola have been confirmed in Liberia, after spreading from neighbouring Guinea, where the deadly virus has killed 78 people.
The two Liberian cases are sisters, one of whom had recently returned from Guinea, officials say.
As concern grows over the outbreak, Senegal has closed its normally busy border with Guinea.
Ebola is spread by close contact and kills between 25% and 90% of its victims.
Senegalese singer Youssou Ndour cancelled Saturday's concert in Guinea's capital Conakry because of the outbreak.
Although he had already travelled to Conakry, he told the BBC it would not be a good idea to bring hundreds or thousands of people together in an enclosed area.
The outbreak began in Guinea's remote south-eastern Forest Region but last week spread to the capital, a sprawling city of two million.
Senegal's Health Minister Awa Marie Coll-Seck said the government decided to close its border with Guinea after confirmation the virus had reached Conakry.
"When it used to be only in the south of Guinea, we didn't do anything special. But now that it's reached Conakry, we believe it's safer to close our borders," she said.
"We have also closed all weekly markets, known as luma, in the south. And we're having some discussions with religious leaders regarding big religious events."
There have also been suspected cases of Ebola in neighbouring Sierra Leone but these have not been confirmed.
The outbreak is believed to have spread to humans from fruit bats, which are a delicacy in parts of south-eastern Guinea.
The government has now banned the sale and consumption of the bats.
It has also urged people to ensure they regularly wash their hands with soap to prevent the virus from spreading.
Discovered in 1976 after an outbreak in the Democratic Republic of Congo, then Zaire, Ebola causes a severe haemorrhagic fever where victims suffer vomiting, diarrhoea and both internal and external bleeding.
Scientists have yet to develop an effective drug or vaccine to fight it.
Outbreaks of Ebola occur primarily in remote villages in Central and West Africa, near tropical rainforests, the World Health Organization says.