UNITED NATIONS — Donors have pledged $250 million to educate over 1 million Syrian children this year but an additional $500 million is needed to fund the program in Turkey, Lebanon and Jordan, the U.N. envoy for global education said Tuesday.

Gordon Brown warned that “death voyages to Europe” will soar in 2016 as long as Syria’s 2 million refugee children and millions more displaced inside the country are exploited and don’t have the opportunity for education.

The organization Girls Not Brides reported that the child marriage rate among Syrian refugee girls has doubled from 12 percent to 26 percent, he said, and a recent survey estimates that one in three refugee children have become child laborers.

“It’s urgent that we provide them education and avoid this exploitation and give these children hope for the future,” Brown told a news conference by audio link from Europe.

When he was Britain’s prime minister in 2009 and head of the Group of 20 leading economies, Brown recalled that $1 trillion had to be raised to support the world economy, so “it ought to be possible to raise $500 million for what is an excellent cause.”

He pledged to try to raise the money at the World Economic Forum in Davos and an international pledging conference for Syria in London, on Feb. 4, among other places.

Brown said Lebanon’s introduction of double-shift classes in school has been the greatest success story, educating over 200,000 refugee children from Syria right now “from a negligible number a few months ago.” The refugee youngsters go to school in late afternoon and evening, after Lebanese children are finished, he said.

The goal, he said, is to double the 200,000 Syrian refugee children now in school in Turkey to over 400,000, increase the number in Jordan from 130,000 to 200,000 in the next few months, and reach 1 million in 2016.

“And by next year every single child refugee would be offered a place in school whether they be in Lebanon, Jordan or Turkey,” Brown said.