The WHO's assistant director general for health security and the environment, Keiji Fukuda, told a Riyadh news conference on Sunday the new virus posed an "important and major challenge" for countries affected and the world generally.

He said experts were still grappling to understand all aspects of the virus and how humans become infected, stressing, however, that "this new virus is not the SARS virus."

"This is a new infection and there are also many gaps in our knowledge that will inevitably take time to fill in," a WHO statement cited Mr Fukuda as saying.

"Of most concern, however, is the fact that the different clusters seen in multiple countries increasingly support the hypothesis that when there is close contact this novel coronavirus can transmit from person-to-person," he said.

"This pattern of person-to-person transmission has remained limited to some small clusters, and so far, there is no evidence that this virus has the capacity to sustain generalised transmission in communities."