Image copyright Reuters Image caption World Health Organization experts say it will take many months to bring the outbreak under control

The UK Foreign and Commonwealth Office says all travel to Sierra Leone, Guinea and Liberia should be avoided - unless essential, due to the Ebola outbreak.

British Airways has suspended flights to Sierra Leone and Liberia and other airlines are taking similar measures.

Such flight restrictions may make it increasingly hard for people working in these areas to leave, the FCO warns.

Meanwhile scientists have announced plans to trial an Ebola vaccine on UK volunteers this September.

The growing restrictions on travel may make it difficult to leave these countries Foreign and Commonwealth Office

Global outbreak

The latest figures show that more than 1,550 people have died from the virus, with more than 3,000 confirmed cases - mostly in Guinea, Liberia and Sierra Leone.

The FCO say British nationals working in these countries "should be aware that the narrowing range of commercial flight options and growing restrictions on travel may make it difficult to leave, particularly at short notice".

The government officials advise people should stay in contact with employers, discussing any help and support they can provide.

This comes as the World Health Organization announces the virus could affect some 20,000 people during the course of the current outbreak which is expected to continue for many months.

But there is no proven vaccine or cure for Ebola.

Researchers at Oxford University will begin a trial of a potential vaccine on healthy UK volunteers once ethical approval is granted.

And if the vaccine works well, the study will extend to The Gambia and Mali.

Image copyright PK Lee/MSF Image caption Health care workers are among those most at risk of catching Ebola

Scientists hope the vaccine will prevent people from catching the disease in affected countries, but will first test the medicine in unaffected populations.

The vaccine consists of a single Ebola virus protein which triggers an immune response once it enters the body - but experts say this cannot cause anyone who is given it to become infected.

In the first part of the study it will be trialled on 60 healthy volunteers and if shown to be safe and working well it will then be administered to 80 volunteers in The Gambia and Mali.

The vaccine could then start to be offered more widely in affected countries during 2015.

'Future epidemics'

Dr Jeremy Farrar, director of the Wellcome Trust, said: "How useful drugs and vaccines might be in complementing existing public health interventions can only be assessed in epidemics.

"The initial safety work we're announcing with our international partners will hopefully make that possible during this crisis and for inevitable future epidemics."

Dr Moncef Slaoui, at GlaxoSmithKline, the company working on the vaccine, said: "Developing a new vaccine is complex with no guarantees of success and it is still early days for our Ebola vaccine candidate.

"But we are encouraged by progress so far and will do the best we can, along with WHO and our partners, to speed up development and explore ways in which the vaccine could contribute to the control of this or future Ebola outbreaks."

Adverts requesting UK volunteers will appear in local papers and on social media, the researchers say.

Each volunteer will be given one dose and be followed up for six months to check for any side-effects.

The Wellcome Trust, the UK Department for International Development and the Medical Research Council are providing a £2.8m grant for this project.

Ebola Virus Disease (EVD)

Image copyright Getty Images Image caption Fruit bats are believed to be a major carrier of the Ebola virus but do not show symptoms