The northern districts of Port Loko and Bombali, and Moyamba in the south, will in effect be sealed off immediately.
Nearly 600 people have died of the virus in Sierra Leone and two eastern districts have been isolated since the beginning of August.
The move follows a three-day nationwide lockdown that ended on Sunday night.
New figures released by the UN World Health Organization show that 2,917 people have died in the current Ebola outbreak in West Africa, with Sierra Leone, Liberia and Guinea worst affected.
Two eastern districts have already been isolated and the extension of the indefinite quarantine means more than a third of Sierra Leone's 6.1 million population now finds itself unable to move freely.
During Sierra Leone's three-day curfew, more than a million households were surveyed and 130 new cases discovered, the authorities say.
Only people delivering essential services can enter and circulate within areas under quarantine.
Skills shortage:
Source: Afri-Dev.Info
Ebola drains weak health systems
In a televised address, the president acknowledged that the blockade would "pose great difficulties" for people.
"[But] the life of everyone and the survival of our country take precedence over these difficulties," he said.
According to WHO, the situation nationally in Sierra Leone continues to deteriorate with a sharp increase in the number of newly reported cases in the capital, Freetown, and its neighbouring districts of Port Loko, Bombali, and Moyamba, which are now under quarantine.
Country | Existing bed capacity | Newly funded beds | Extra beds still required |
---|---|---|---|
Source: WHO, 24 September 2014 |
|||
Guinea |
180 |
0 |
40 |
Liberia |
315 |
440 |
1,550 |
Sierra Leone |
323 |
297 |
532 |
The BBC's Umaru Fofana in Freetown says Port Loko is where two of the country's major iron ore mining companies operate and the restrictions are likely to hamper business.
The WHO said despite efforts to deploy more health workers and open new Ebola treatment centres in the worst-affected countries, there was still a significant lack of beds in Sierra Leone and Liberia, with more than 2,000 needed.
The situation in Guinea appeared to be stabilising, with up to 100 new confirmed cases reported in each of the past five weeks, but it was still of grave concern, it said.
A high-level meeting is due be held on the sidelines of the UN General Assembly in New York about the Ebola crisis.
Ebola virus disease (EVD)