Freetown (AFP) - Sierra Leone said Monday it had launched an emergency operation to prevent a serious spread of Ebola after two patients escaped for several hours from a treatment centre.

A highly-infectious 32-year-old woman and eight-year-old girl sparked a nationwide alert by absconding together from the clinic on the outskirts of the capital Freetown on Saturday.

The pair, who are not related, were tracked down at an undisclosed location in the evening and returned to the clinic after radio appeals were made to the public, the National Ebola Response Centre told reporters in Freetown.

"Our contact tracers and surveillance officers are meanwhile tracking the level of contacts the two would have made during the period of their escape," a spokesman said.

The worst outbreak in history has seen 27,600 infections in Guinea, Liberia and Sierra Leone, of which 11,253 have been fatal, according to official data largely deemed to be an underestimate.

Sierra Leone has seen almost half of the total caseload, reporting 3,941 deaths as of Wednesday last week, and is currently treating 10 patients.

The World Health Organization (WHO) reported in June that a retreat of the virus "that was apparent throughout April and early May has stalled" in Sierra Leone.

President Ernest Bai Koroma announced in early June he was imposing curfews in the worst-hit parts of the northwestern districts of Kambia and Port Loko.

They were the only two areas at the time still reporting new infections, although the capital Freetown has since seen its own flare-up.

The curfews had been due to end earlier this month but have been extended indefinitely.