LONDON (AFP) - Malala Yousafzai told world leaders they were failing Syria's children, as the Nobel Peace Prize winner spent her 18th birthday on Sunday on the Syrian border.

As she became an adult, the teenager, who was shot by militants in her native Pakistan for campaigning for girls' rights, opened a school for more than 200 Syrian girls living in refugee camps in Lebanon's Bekaa Valley.

The Malala Yousafzai All-Girls School will offer education and skills training to girls aged 14 to 18.

"I am honoured to mark my 18th birthday with the brave and inspiring girls of Syria," Yousafzai said in a statement received in London.

"I am here on behalf of the 28 million children who are kept from the classroom because of armed conflict.

"Their courage and dedication to continue their schooling in difficult conditions inspires people around the world and it is our duty to stand by them.

"On this day, I have a message for the leaders of this country, this region and the world: You are failing the Syrian people, especially Syria's children. This is a heartbreaking tragedy - the world's worst refugee crisis in decades."

Lebanon is hosting nearly 1.2 million registered Syrian refugees, though the total number in the country may be even higher.

The influx has placed strains on Lebanon, which has just four million citizens.

The Lebanese government has prevented the establishment of official refugee camps, giving rise to informal shanties known as "tented settlements" in rural areas.

Malala was flown to Britain for treatment after the Pakistani Taleban tried to kill her in October 2012, and now lives permanently in Britain with her family.