"Drastic action" is needed to contain the spread of deadly Ebola in West Africa, according to the World Health Organization (WHO).
Nearly 400 people have died in the outbreak which started in Guinea and has spread to neighbouring Sierra Leone and Liberia.
It is the largest outbreak in terms of cases, deaths and geographical spread.
The WHO said it was "gravely concerned" and there was potential for "further international spread".
The outbreak started four months ago and is continuing to spread.
So far there have been more than 600 cases and around 60% of those infected with the virus have died.
Ebola, a haemorrhagic fever, has no cure and is spread by contact with the fluids of infected people or animals, such as urine, sweat and blood.
Most of the deaths have been centred in the southern Guekedou region of Guinea.
The WHO has sent 150 experts to the region to help prevent the spread of the virus but admits " there has been significant increase in the number of daily reported cases and deaths".
Dr Luis Sambo, the WHO's regional director for Africa, said: "This is no longer a country-specific outbreak, but a sub-regional crisis that requires firm action.
"WHO is gravely concerned of the on-going cross-border transmission into neighbouring countries as well as the potential for further international spread.
"There is an urgent need to intensify response efforts...this is the only way that the outbreak will be effectively addressed."
The charity Medecins Sans Frontieres (MSF) has already warned that the Ebola outbreak is out of control.
It says the epidemic will spread further unless there is a stronger international response.
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