The WHO says the epidemic is the world's largest and fastest moving.
Around 310,000 suspected measles cases have been reported since the start of 2019, the WHO says.
The Congolese government and the WHO launched an emergency vaccination programme last September.
More than 18 million children under five were vaccinated across the country in 2019, the WHO says.
But poor infrastructure, attacks on health centres and a lack of access to routine healthcare have all hindered efforts to stop the spread of the disease.
Every one of the country's 26 provinces has reported cases of measles since the outbreak was declared in June last year.
DR Congo is also experiencing the second most deadly outbreak of Ebola the world has ever seen but this has killed less than half as many people as measles in the country.
"We are doing our utmost to bring this epidemic under control," said Dr Matshidiso Moeti, WHO Regional Director for Africa.
"Yet to be truly successful we must ensure that no child faces the unnecessary risk of death from a disease that is easily preventable by a vaccine. We urge our donor partners to urgently step up their assistance."
The WHO says an extra $40m (£30m) is required to extend the vaccination to children between six and 14 years and strengthen outbreak response.
What is measles?
Measles is a virus that initially causes a runny nose, sneezing and fever.
A few days later it leads to a blotchy rash that starts off on the face and spreads across the body.
Most people will recover, but measles can cause life-long disability. It can be deadly, especially if it causes pneumonia in the lungs or encephalitis (swelling in the brain).
It is estimated that a global total of 110,000 people die from measles each year.