(CNN) Measles has killed more than 6,000 people in the Democratic Republic of Congo (DRC), the World Health Organization has said.

In a statement Tuesday, the organization called on international partners and agencies to increase resources to fight what it called the "world's worst measles epidemic."

Around 310,000 suspected measles cases have been reported in the DRC since last year and a shortage of funds remained a "huge impediment" to efforts to curb the outbreak, the WHO said.

The WHO said even though the agency and its international partners have vaccinated 18 million Congolese children under the age of 5 against the disease, routine immunization coverage is still low across the country. It said 25% of reported cases in the country occurred in children under 5, who are most vulnerable to the vaccine-preventable disease.

The organization said that although it has mobilized $27.6 million, it needed a further $40 million to include children aged between 6 and 14 in vaccination programs while also bolstering other health responses to the outbreak.

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