In Europe, experts blamed the problem in part on complacency and misinformation about a vaccine proven to be both effective and safe.
Martin Friede, WHO's director of immunisation, vaccines and biologicals told reporters that "supposed experts making accusations against the vaccine without any evidence" has had an impact on parents' decisions.
He specifically cited medically baseless claims linking the measles vaccine to autism, which have been spread in part on social media by members of the so-called "anti-vax" movement.
But cases have also spiked in Latin America, partly due to "a collapsing health system in Venezuela," the head of the vaccine alliance Gavi, Seth Berkley, said in a statement.
A crippling political and economic crisis in Venezuela has triggered massive inflation, with hospitals struggling to maintain stocks.
"What is more worrying than the increase in the cases reported is that we are seeing sustained measles transmission in countries that had previously not seen measles transmission for many years," Friede said.
Source: Getty images/South_agency
"This suggests we are actually regressing."
Multiple countries - notably Germany, Russia and Venezuela - have had their measles elimination certificate withdrawn over the last 12 months.
A country loses its measles elimination status when "the same type of virus has been circulating for more than 12 continuous months," according to WHO.
'Not rocket science'
WHO stressed that the overall global fight against measles had shown impressive results this century. In 2000, there were more than 850,000 cases reported worldwide, compared to 173,000 last year.
That progress made the recent setbacks all the more frustrating, said WHO immunisation expert Ann Lindstrand.
"We have a safe and effective vaccine," she told reporters. "This is not rocket science, we know what to do."
According to WHO guidelines, preventing measles outbreaks requires 95 percent coverage of the first dose of the vaccine.
Global coverage has stalled at 85 percent for several years, but the figure is lower in poorer regions like Africa, which had a coverage rate of 70 percent in 2017.
Measles is a highly contagious disease, which can cause severe diarrhoea, pneumonia and vision loss and can be fatal in some cases.