(CNN) More people have died from measles in the Philippines so far this year than in all of 2018, with experts warning about 2.6 million children remain at risk in the country.

The outbreak has killed 261 people this year alone -- and most of the victims were children aged under five, the International Federation of Red Cross and Red Crescent Societies (IFRC) found.

This compares with 202 total deaths in 2018. And that was a 547% increase on the previous year.

"It is completely unacceptable that children are still dying from measles in 2019," said Richard Gordon, chairman and CEO of the Philippines Red Cross.

The IFRC, with the Philippines' Department of Health, is now aiming to immunize all unvaccinated children across seven of the "hardest-hit regions" over the next 12 months.

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