The World Health Organization has warned that dramatic steps are needed to fight the deadliest Ebola outbreak on record, calling an 11-nation meeting to address the crisis.

"As the number of deaths and cases of Ebola virus continues to rise in Guinea, Liberia and Sierra Leone, the World Health Organization is warning that drastic action is needed," the UN agency said on Thursday in a statement.

As of Sunday, 635 confirmed, probable and suspected cases of Ebola have been reported, including 399 deaths, making the ongoing outbreak the largest ever "in terms of the number of cases and deaths, as well as geographical spread," WHO said.

The agency was now "gravely concerned (by) the on-going cross-border transmission into neighbouring countries as well as the potential for further international spread," said WHO's regional director for Africa, Dr Luis Sambo.

"This is no longer a country specific outbreak but a sub-regional crisis that requires firm action by governments and partners," Sambo warned.

Since west Africa's first-ever epidemic of the deadly haemorrhagic fever emerged in Guinea in January, WHO has sent in more than 150 experts to help tackle the crisis.

But despite the efforts of the WHO and others, there had been a "significant increase" in the number of cases and deaths reported each day for the past three weeks, it said.

To address the growing crisis, the WHO said it would convene a meeting of the health ministers from 11 countries in Accra, Ghana on July 2 and 3 "to discuss the best way of tackling the crisis collectively as well as develop a comprehensive inter country operational response plan."

WHO has described the epidemic as one of the most challenging since the virus was first identified in 1976 in what is now the Democratic Republic of Congo.

That outbreak, until now the deadliest, killed 280 people, according to WHO figures.

Ebola is a tropical virus that can fell its victims within days, causing severe fever and muscle pain, weakness, vomiting and diarrhoea - in some cases shutting down organs and causing unstoppable bleeding.

No medicine or vaccine exists for Ebola, which is named after a small river in the DRC.