Sierra Leone's President Ernest Bai Koroma has widened a quarantine to include another one million people in an attempt tocurb the spread of Ebola.
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 where two eastern districts are already blockaded.
Meanwhile, US President Barack Obama has said the world needs to act faster to halt the West Africa Ebola outbreak.
"There is still a significant gap between where we are and where we need to be," Mr Obama told a high-level United Nations meeting on Ebola.
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.
'Great difficulties' Mr Koroma's announcement follows a three-day nationwide lockdown that ended on Sunday night.
Two eastern districts have been isolated since the beginning of August 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.
President Koroma said the move had been a success but had exposed "areas of greater challenges", which was why other areas were being quarantined.
Only people delivering essential services can enter and circulate within areas under quarantine.