Tedros Adhanom Ghebreyesus' comments came during a video press conference from WHO headquarters where he noted the coronavirus in Western Europe appears to be stable or declining.
In Africa, Central and South America and Eastern Europe where numbers are low, there are worrying upward trends, said Tedros.
He said most countries are in the early stages of COVID-19 but some that were affected early are now seeing a resurgence in cases.
"Make no mistake: we have a long way to go. This virus will be with us for a long time," said Tedros. "There's no question that stay-at-home orders and other physical distancing measures have successfully suppressed transmission in many countries. But this virus remains extremely dangerous."
He said early evidence suggests most of the world's population remains susceptible.
"That means epidemics can easily re-ignite," said Tedros.