Governments and aid agencies have been unable to account for millions of dollars in school aid for Syrian refugees, frustrating efforts to meet schooling needs for children in Lebanon, Jordan, and Turkey, according to a rights group.

More than 500,000 school-age Syrian refugees in the three countries were out of school last year, in part because of severe funding gaps, says the report released by the New York-based Human Rights Watch (HRW) on Thursday.

There are at least 1.6 million Syrian children refugees in the region.

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In Lebanon, classes for Syrian refugee children are packed, and students attend school in half-day shifts to free up classroom space for more enrolment.

Even so, more than half the school-age Syrians in the country are not attending school, according to Lisa Abou Khaled, a communications officer for the UN refugee agency UNHCR.

The challenges include pulling impoverished children away from work, finding money to pay for their transportation and supplies, and helping students feel safe and comfortable in schools.

HRW says late or missing aid has compounded those problems.

The US government aid agency USAID, for example, reported it had given $248m in school aid to Jordan last year, but Jordan's government only reported receiving $13m.

A separate USAID database only tracked $82m in disbursements that year.

Aid agencies in Lebanon are trying to fill a $25m budget deficit to get students back in school this year, Abou Khaled said.

Short of aid pledges

HRW says donor countries fell $97m short of their pledges to provide $250m for schooling in Lebanon in 2016.

The six largest donors are the EU, the US, Germany, the UK, Japan, and Norway.

Simon Rau, an HRW researcher and one of the authors of the report, says more transparency is needed to identify the funding gaps.

"Donors and host countries have promised that Syrian children will not become a lost generation," said Rau.

"But this is exactly what is happening."