Guinea and Sierra Leone reported 35 new Ebola cases in the past week, four times as many as the week before, in a reminder that the virus "will not go quietly," a top World Health Organization official said on Tuesday.

"It will take an extraordinary effort to finish the job," the WHO's special representative for Ebola, Bruce Aylward, told a briefing attended by health ministers.

HEALTH-EBOLA/LEONE

The Ebola virus has shown how easy it is for a single cross-border traveller to reignite the epidemic, the World Health Organization says. (Baz Ratner/Reuters)

"With the start of the rainy season today, the doubling of effort will be that much more difficult," he added, referring to increased logistical challenges.

Ghana president John Dramani Mahama said on Tuesday that the West African regional bloc ECOWAS expected zero Ebola infection in West Africa "in the next couple of months, if not weeks."

The 35 cases in the week to May 17 were in six districts of Guinea and Sierra Leone, with most in Guinea, Aylward said. Nine were confirmed the previous week.

Liberia, the other worst-hit country, was declared Ebola-free earlier this month.

"The virus has shown how easy it is for a single cross-border traveller or unsafe burial to reignite the epidemic again," said WHO Director-General Margaret Chan.

The WHO said on Monday it is setting up a $100 million contingency fund to ensure that it will not be "overwhelmed" by a major crisis again as it was with Ebola, which has killed more than 11,000 people since December 2013.

The United States, Britain and the European Union, all top donors, called for deep WHO reforms.

"We need to keep up pressure and momentum both to get to zero [cases] and help with the rebuilding after that," said U.S. health secretary Sylvia Mathews Burwell. "We need a WHO that can put the right people in place to deal with these issues."

Sally Davies, chief UK medical officer, and the European Union's Claus Sorensen said problems in sharing epidemiological data had slowed responses during the outbreak.

Guinea's health minister, Remy Lamah, said authorities were searching out cases but traditional cultural practices, such as washing dead bodies, were still proving hard to overcome.

"There are certain cases of dissent with respect to measures taken in parts of the country but it is going down," he said.

Sierra Leone's chief medical officer Brima Kargbo said there were "signs that our strategy is working" to get to zero cases, such as fewer people dying in quarantine homes.

"We need continued vigilance with a focus on hotspot districts," he said.

Tim Evans, health director at the World Bank, said that Ebola was also a development crisis. "Getting seeds for farmers, reopening schools, embracing infrastructure projects that had been put on delay are critical parts of the recovery."