|A WHO statement said that Sierra Leone accounted for 14 deaths and 34 of the 50 new cases|
Health officials in West Africa say 25 more people have died from Ebola since 3 July, taking the total number of deaths to 518.
The World Health Organization (WHO) said 50 new cases of the deadly disease had also been reported.
A WHO spokesman said health workers were struggling to contain the outbreak in Sierra Leone, Liberia and Guinea.
On Monday, a doctor in Ghana said preliminary tests on a US citizen showed he did not have the disease.
But further tests are now being carried out.
The man had recently visited Sierra Leone and Guinea and was quarantined after showing signs of the virus.'A mixed picture'
In a statement on Tuesday, the WHO said the latest figures from health ministries in Sierra Leone, Liberia and Guinea showed a total of 844 cases since the epidemic began in February.
Guinea's ministry reported two deaths since 3 July but no new cases in the past week, the WHO said, calling the situation in the affected region of West Africa a "mixed picture".
It said Sierra Leone had accounted for 34 of the new cases and 14 deaths, while Liberia reported 16 new cases and 9 deaths.
"These numbers indicate that active viral transmission continues in the community," the statement said.
WHO spokeswoman Fadela Chaib said the two main modes of transmission were people caring for sick relatives at home and people attending funerals of victims.
"If we don't stop the transmission in the several hotspots in the three countries we will not be able to say that we control the outbreak," she said.
The BBC's Tulip Mazumdar says experts believe the key to stopping the spread of the virus is making sure affected communities understand it better.
Last week, health ministers from 11 West African countries adopted a common strategy to fight the outbreak.
At an emergency meeting in Ghana last Thursday, ministers promised better collaboration to fight what has become the world's deadliest outbreak to date.
Under the new strategy, the WHO will open a sub-regional control centre in Guinea to co-ordinate technical support.Ebola virus disease (EVD)
- Symptoms include high fever, bleeding and central nervous system damage
- Fatality rate can reach 90%
- Incubation period is two to 21 days
- There is no vaccine or cure
- Supportive care such as rehydrating patients who have diarrhoea and vomiting can help recovery
- Fruit bats are considered to be the natural host of the virus