All non-essential outgoing travel to the affected countries has been banned.

Senegal also said it was suspending flights with Ebola-affected countries, and closing the border with Guinea.

Cameroon and the Ivory Coast earlier imposed travel bans, despite World Health Organization warnings not to.

South African nationals will be allowed to re-enter the country when returning from high-risk countries, but will undergo strict screening, the health ministry said on Thursday.

Usual screening procedures are in place for those who travel between Nigeria, Kenya and Ethiopia, which have been defined as medium-risk countries.

South Africa has experienced two Ebola scares in recent weeks, involving passengers arriving from Liberia and Guinea, but the country has so far remained Ebola-free.

Johannesburg has one of the major transit airports, connecting southern Africa with the rest of the continent. Several airlines, including British Airways and Emirates Airlines, have stopped flights to some of the affected countries.

Meanwhile, Senegal's interior ministry announced on Friday it was closing the land borders with Guinea "once again".

Senegal shut its border with Guinea for the first time in March after the virus reached the capital Conakry.

In a statement, it said this extends to "air and sea borders for aircraft and ships from the Republic of Guinea, Sierra Leone and Liberia".

Separately, the African Union (AU) announced plans to send a special team to the four affected countries.

The six-month-long operation, involving volunteer doctors, nurses and medical personnel, will cost about $25m (£15m) and begin immediately, the AU said in a statement.

Aid workers and medical staff are most exposed to the virus, and have been most at risk of becoming infected. Two US aid workers were discharged from hospital on Tuesday, after recovering from the virus they contracted in Liberia.

The supply of the experimental drug used to treat the couple, ZMapp, has been exhausted.

As Ebola has no known cure, it is being controlled by isolating victims and those who have come into contact with them.