"What we are seeing still is a geographic expansion across countries."
H1N1 cases double
The WHO says the number of H1N1 cases has doubled in the last three weeks, with most of the deaths from the virus concentrated in the Americas.
But the spread of the disease into other continents continues, as authorities in Britain announced more than 100,000 infections were diagnosed in the country last week.
Hartl said the pandemic strain is causing mainly mild symptoms, but he warned that the virus could mutate with the onset of colder temperatures in the northern hemisphere.
"We don't know how the virus will change going forward," he said.
"We do have to be aware that there could be changes and we have to be prepared for those."
Health officials and pharmaceutical companies are looking into ways to speed up production of an H1N1 vaccine before the northern hemisphere enters its flu season in the fall.
Hartl said he expects the first doses of a vaccine to be available for use in early autumn of the northern hemisphere.
But Dr Keiji Fukuda, the assistant director-general of health security and environment at the WHO, warned that the situation will continue to change.
"Right now we're trying to get a handle on what is the real effect of the infection with this particular virus," he told Al Jazeera on Friday.
"We're dealing with a very new phenomenon, a new virus, a virus which is spreading to most countries in the world and over the next year, probably all countries," he said.
"In the current situation, most people who are getting infected develop mild illnesses ... they get better. However, what is of concern is we continue to see people die from the virus.
"What we want to know is, is this pattern going to change?"