By Zoe Tabary

LONDON (Thomson Reuters Foundation) - Education is a leading, and hidden, casualty of war, with some 3 million children out of school in Syria alone, said Irina Bokova, a Bulgarian diplomat hoping to become the next head of the United Nations, on Tuesday.

World leaders must act to improve education in nations torn by war, Bokova said at an appearance in London.

"Only 2 percent of humanitarian aid goes to education," she said. "It's the hidden face of war."

About 263 million children worldwide do not attend school, according to the U.N. Educational, Scientific and Cultural Organization (UNESCO).

More than a third of children out of school live in countries hit by armed conflict, UNESCO said.

Bokova, former foreign minister of Bulgaria and UNESCO director-general since 2009, is one of ten candidates hoping to succeed U.N. Secretary-General Ban Ki-moon, who will step down at the end of 2016 after serving two five-year terms.

In the most recent balloting, held last week, former Portuguese Prime Minister Antonio Guterres led the race, followed by Slovakia's Foreign Minister Miroslav Lajcak and a third-place tied between Bokova and former Serbian Foreign Minister Vuk Jeremic, diplomats said.

The 15-member Security Council will hold secret ballots until a consensus is reached.

Civil society groups and nearly a third of the 193 U.N. member states pushed for the first woman secretary-general, but those hopes appear to be fading.

Speaking at the Chatham House, a policy institute in London, Bokova said prevention of conflict and peace building should be the main focus of the next U.N. chief and said the global body should invest in diplomacy as well as peace keeping missions.

"Any peace keeping mission should be viewed as part of a larger political strategy," she said.

She listed violent extremism as the biggest threat to world peace and security but warned against fighting terrorism at the expense of democracy.

"The two are not mutually exclusive," she said.

She acknowledged the U.N.'s limitations but said: "I don't see any better framework out there to find solutions to the world's problems.

"Multilateralism is tedious, slow, but diplomacy is the only way so it has to go on," she said.

Diplomats have said the Security Council aimed to recommend a candidate to the U.N. General Assembly for election by October.

Ultimately, the five permanent council members - the United States, Russia, Britain, China and France - must agree on a candidate.

(Reporting by Zoe Tabary, Editing by Ellen Wulfhorst; Please credit the Thomson Reuters Foundation, the charitable arm of Thomson Reuters, that covers humanitarian news, women's rights, trafficking, land rights and climate change. Visit