The three West African countries at the heart of an Ebola epidemic recorded their first week with no new cases since the outbreak was declared in March 2014, the World Health Organization (WHO) said on Wednesday.

The U.N. agency said that more than 11,000 people have died in Guinea, Liberia and Sierra Leone in the world's worst known occurrence of Ebola, but there were no new cases in the week to Oct. 4.

All contacts of Ebola cases in Sierra Leone have now been followed for 21 days without falling sick, suggesting the country might soon be free of the disease, WHO said in a situation report. The nation released its last known Ebola patient on Sept. 28 and must wait another 42 days until it can be declared free of the disease.

New cases of Ebola have dwindled sharply this year, but WHO said there was still a risk of resurgence.

"Over 500 contacts remain under follow-up in Guinea, and several high-risk contacts associated with active and recently active chains of transmission in Guinea and Sierra Leone have been lost to follow-up," the report said.

“There remains a near-term risk of further cases,” the report added.

Liberia was declared Ebola free for a second time on Sept. 3, after a flare-up in June but remains under heightened surveillance. Guinea's most recent cases were recorded on Sept. 27.

Al Jazeera and wire services