More than half of the nearly 500,000 Syrian children registered as refugees in Lebanon are missing out on formal education and a government campaign to increase enrolment has failed to reach its target, Human Rights Watch said on Tuesday.

There are more than 1 million Syrian refugees in Lebanon, making up a quarter of the country's population. Since Syria's civil war began in 2011, nearly 5 million Syrians have fled, including to Turkey, Europe and Jordan.

Lebanon's education ministry, supported by U.N. and international donors, launched a campaign last year to provide schooling for 200,000 Syrian children. This included setting up a second, afternoon shift of teaching to cope with the higher numbers.

But Human Rights Watch said that nearly 50,000 of those places went unused.

Many Syrian children failed to enroll or dropped out during the year because of inability to afford transport to school or school supplies, a report released by the New York-based watchdog said.

Others were deterred by arbitrary enrolment requirements imposed by individual schools, corporal punishment and language barriers, with classes taught in English and French as under the Lebanese system.

Less than 3 percent of Syrian refugee children in Lebanon aged 15-18 enrolled in public secondary schools in the last academic year, the report said.

HRW urged the education ministry to allow NGOs to provide non-formal education to help fill the gap, and to revise residency requirements which it said were also preventing Syrians' access to education by restricting freedom of movement and exacerbating poverty.

Syrians must pay $200 a year for legal stay in Lebanon, but many cannot afford the fee, or confine themselves to their camps and do not approach authorities to renew expired residency papers because they fear arrest.

Restricted movement for Syrian adults also means many families rely on their children for income, aid agencies say.