Police in Ebola-hit Sierra Leone have raided a funeral and arrested 13 people suspected of organising an unsafe burial, risking spreading the disease, officers said.

Police superintendent Da Samah said "heavily armed" police arrived just in time to stop a 50-year-old man being interred on the outskirts of Freetown after they were tipped off about the ceremony.

"We stopped the burial and we have put out an alert for an ambulance which eyewitnesses said brought the corpse to the cemetery," he said.

He said those present at the funeral on Thursday last week were arrested because they had no burial permit or other required documents.

Health authorities in Freetown have recorded 3,832 of more than 10,000 deaths in the outbreak, which began in southern Guinea in December 2013 before spreading to Liberia and Sierra Leone.

Ebola, one of the deadliest known viruses, is spread only through direct contact with the bodily fluids of an infected person showing symptoms such as fever or vomiting, or the recently deceased.

Palo Conteh, the national Ebola response chief, said traditional funeral rights involving contact with the dead remained the biggest driver of Ebola transmission.

Unsafe burials were common at the start of the epidemic but the latest data from the World Health Organisation (WHO) show just one was reported in Sierra Leone in the week to March 29.

Alimamy Kamara, head of the National Ebola Response Centre (NERC) in charge of Freetown and the surrounding area, said last week that people organising unsafe burials risked two-year jail terms.

"People should adhere to the government policy of safe and dignified burials so that we can end the high rate of transmission of the Ebola virus," he said.

The NERC said later on Tuesday that health authorities had wrongly reported Ebola as the cause of a baby's death in the eastern district of Kailahun, which was declared free of the virus weeks ago.

Villagers in the in the Njaluahun tribal chiefdom were dismayed when the government said on Monday a nine-month-old boy had tested positive after his death.

The NERC said in a statement however an investigation had found that the "child in question is not an Ebola case as the sample in question is not from the child".

It did not say how the error had happened but concluded that there was "no evidence to suggest that there is an ongoing Ebola transmission in Kailahun".

Meanwhile officials in the United States said an American healthcare worker who fell ill with Ebola in Sierra Leone had improved and was now in good condition at a hospital near Washington DC.

The man, whose identity has not been revealed, worked for the global medical charity Partners in Health, and was repatriated on March 14.

AFP