A four-day nationwide lockdown announced by the Sierra Leone government in a bid to contain the biggest ever outbreak of Ebola could instead exacerbate the spread of the disease, aid agencies have warned.

From 18 to 21 September people across the west African nation will not be allowed to leave their homes, a senior official in the president's office said on Friday.

But Médecins sans Frontières (MSF) raised concern about the drastic step, warning that it could lead people to try to conceal infections from the authorities.

A spokeswoman said: "It has been our experience that lockdowns and quarantines do not help control Ebola as they end up driving people underground and jeopardising the trust between people and health providers. This leads to the concealment of potential cases and ends up spreading the disease further."

The move was intended to allow health workers to identify and isolate new cases to prevent the disease from spreading further, said Ibrahim Ben Kargbo, a presidential adviser on the country's Ebola task force.

"The aggressive approach is necessary to deal with the spread of Ebola once and for all," he said.

There were riots in Monrovia, Liberia, after the government quarantined a community in one of the poorest areas of the capital last month.

The MSF spokeswoman said the spread of virus would be more effectively controlled with more specialist care beds. "What Sierra Leone and Liberia both urgently need are more beds in case management centres, and they need them now," she said.

So far, more than 3,685 people have been infected in west Africa, with 1,841 deaths from the virus recorded in Sierra Leone, Liberia, Guinea and Nigeria since March, according to the latest World Health Organisation (WHO) figures.

As of 4 September, Sierra Leone's health and sanitation ministry had recorded 404 deaths. Last week, the WHO warned that there could be another 20,000 cases across west Africa before the outbreak is stopped.

Dr Mohamed Yilla, a Freetown obstetrician and country director of the MamaYe maternal health programme, told the Guardian the lockdown was a good idea but said it would be a challenge to enforce.

"If it is handled properly and we get it right, it will yield some significant results," he said.

He said the outbreak was spreading because infected people were not going to hospital for treatment but were staying at home and passing the virus on to other family members.

This lack of trust in hospitals and doctors was also causing additional problems, he added.

"People still have heart attacks, children are still getting sick and mothers are still giving birth, but they are not showing up to the hospitals. We now have the potential for things like measles to start creeping up again. We have to be prepared for the bigger health crisis we have on our hands."

Yilla said the immediate problem would be logistical. If 1,000 new potential cases were identified during the lockdown, there needed to be enough ambulances to transport them, staff to deal with blood screening and the correct facilities in hospitals.

Kargbo has said 21,000 people would be recruited to enforce the lockdown. Thousands of police officers and soldiers have already been deployed to enforce the quarantining of towns in Sierra Leone's worst-hit regions, near the border with Guinea.

Tom Dannatt, founder of Street Child, a charity which employs 650 people in Sierra Leone and Liberia, also raised concern over the scale of the lockdown.

He said: "Can you imagine telling everyone in this country you have to stay at home for three days?"

He also expressed concerned about the country's street children, many of whom live in food markets which will be closed. "Where will they go?" he asked.

But Dannatt said he hoped the measure would make the international community wake up to the severity of the outbreak. "It will highlight the fact Ebola is touching everyone. It's not just the thousand who have contracted it," he said.

"I can see in six or nine months' time, people will say how did we get to this. But this is like watching a train coming down the tracks."

Sierra Leone has confirmed 1,179 infections, including British nurse Will Pooley who has completely recovered from the virus after being airlifted out of the country by the RAF for treatment at the Royal Free hospital in London.

Pooley worked in the government hospital in Kenema, the third largest city in Sierra Leone, which has been quarantined for some time and is the centre of the outbreak.

More than 20 infected health workers have died in Sierra Leone since the start of the outbreak in March.

Earlier this week the director general of the WHO, Dr Margaret Chan, called for a massive global response to the outbreak and suggested it would take at least six to nine months to halt the virus and cost more than $600m (£365m) .