Ebola outbreak 'presents challenges'
31 July 2014 Last updated at 01:19 BST
The number of people killed by the virus in West Africa has now reached 672, according to new UN figures.
The US humanitarian organisation Peace Corps said it was withdrawing 340 volunteers from Liberia, Sierra Leone and Guinea because of the spread of the virus.
Dr Stephan Monroe from the Centers for Disease Control and Prevention said the outbreak was unusual and logistically challenging to break the chains of transmission.