The World Health Organisation has urged governments not to impose blanket bans on trade and travel on Ebola-affected countries after Kenya joined a growing number of countries and airlines severing links to three west African states.

The WHO has already said that the risk of Ebola transmission from air travel is low, but the level of fear is so high that several airlines have disregarded the UN agency's advice. The disease has already killed at least 1,145 people across west Africa this year.

"The scale of the outbreak is much larger than anything ever seen before," said Gregory Hartl, a WHO spokesman. "It is an obvious source of concern and it is not to be underestimated, but we must take measures commensurate with the risk. What you don't want to do is to take blanket measures to cut off travel and trade."

Despite such advice, Kenya is the latest country to jump on the bandwagon by declaring a travel ban on Saturday. From midnight on Tuesday, people travelling from or through Sierra Leone, Guinea and Liberia will not be allowed to enter the east African country, said Kenya's health ministry.

Nigeria, which allows entry to health professionals and Kenyans returning from those countries, was not included in the ban. The outbreak began in the forested zone on the borders of Guinea, Sierra Leone and Liberia earlier this year, and spread to Nigeria last month.

Ebola Travel Bans WEB2

The WHO last week said the outbreak had been vastly underestimated and will require "extraordinary measures, on a massive scale" to contain it. Médecins sans Frontières, the medical charity, said the disease was spreading "faster than we can respond to", and accused the WHO of being too slow to react.

Following the Kenyan government's decision, Kenya Airways said it would suspend flights to Liberia and Sierra Leone, abruptly reversing its announcement on Friday that it would continue flying to those countries. The airline had been accused by medical experts and Kenyan politicians of putting profits ahead of prudence amid fears that its flights would spread the virus. The airline flies more than 70 flights a week to west Africa.

The airline's chief executive, Titus Naikuni, had argued at a news conference on Friday that its flights actually helped to contain the Ebola outbreak by transporting medical staff, supplies and equipment to west Africa. But doctors representing the Kenya Medical Association had asked Kenya Airways to suspend flights to the four countries affected by Ebola "until things stabilise". MPs also called on the carrier to halt its west African operations.

Several airlines have already suspended flights to Sierra Leone, Guinea and Liberia, including British Airways, Emirates Airlines, Arik Air and ASKY Airlines. Nigeria became the fourth Ebola-affected country after a Liberian-American man flew to Lagos on an ASKY flight and infected several people in the city before he died.

Officials in Cameroon, which borders Nigeria, announced on Friday it would suspend all flights from all four Ebola-affected countries. Korean Air announced on Thursday it would temporarily halt its service to Kenya despite there being no cases of Ebola in the country.

Nigeria has withdrawn its athletes from the Youth Olympics in Nanjing, China, as a result of the outbreak; the games opened on Saturday. The International Olympic Committee has barred athletes from Ebola-hit countries from competing in pool events and combat sports, affecting three athletes.

Hartl said action commensurate with the risk of Ebola spreading through air travel has already been taken in the three most-affected countries: the screening of passengers.

"We know it's happening. It makes sense at the departure end and there is nothing to stop screening at the incoming end. These are much better measures. You do not need a sledgehammer to crack a nut."

Screening involves visual observations and asking passengers questions such as whether they have had known contact with Ebola cases and whether they feel ill or feverish. No one has been stopped from travelling since the screenings were introduced, said Hartl, who pointed out that the Liberian-American who died in quarantine in Lagos had left before screening was in place. But screening in his case might not have worked as he had left with no symptoms, and only reported feeling unwell on arrival in Lagos.

Ebola-affected countries are suffering economically as international airlines restrict flights, companies scale down regional operations and commercial trade is put on hold, said the World Bank. The bank, along with the International Monetary Fund, reduced this year's economic growth estimate for Guinea to 3.5%, down from their original projection of 4.5% growth.