"Although the total number of cases outside China remains relatively small, we are concerned about the number of cases with no clear epidemiological link, such as travel history to China or contact with a confirmed case," he said.
The increase in cases in Iran, South Korea and Italy "is also a matter of concern and how the virus is now spreading to other parts of the world," he added.
The director general reiterated that while there is still a chance to contain the virus, "the window of opportunity is narrowing."
"Our biggest concern continues to be the potential for (the virus) to spread in countries with weaker health systems."
While 80% of patients exhibiting mild symptoms will recover, 20% of patients with "severe or critical disease, ranging from shortness of breath to septic shock and multi-organ failure," require "respiratory support machines that are, as you know, in short supply in many African countries," he added.
The WHO has supplied laboratory test kits and 30,000 sets of protective equipment to a number of priority countries in Africa, many of which have "direct links to China or their high volume of travel with China."
"We’ve also published a Strategic Preparedness and Response Plan, with a call for $675 million to support countries, especially those which are most vulnerable," he said.
There are over 77,700 cases globally, and at least 17 deaths outside mainland China.