The youngest person to ever win the prestigious Nobel Peace Prize for her advocacy for girls' education has chosen to celebrate her 18th birthday in Lebanon in an extraordinary way that would help other people: by opening a school for Syrian refugee girls in the Beka'a Valley.

On her first day as an adult, Nobel winner Malala Yousafzai asked world leaders to invest in "books not bullets."

"I decided to be in Lebanon because I believe that the voices of the Syrian refugees need to be heard and they have been ignored for so long," Malala said on Sunday as she opened the school in an informal refugee camp.

"Today on my first day as an adult, on behalf of the world's children, I demand of leaders we must invest in books instead of bullets," she said in a speech.

Malala was shot in the face at close range by Taliban fighters while she was on a school bus in her native Pakistan in 2012 because she pushed for girls' right to education. She miraculously survived the shooting and continued her campaign. She was eventually named as the winner of the Nobel Peace Prize.

The Malala Fund, a non-profit group that aids local education projects, financed the school in the Bekaa Valley, close to the Syrian border.

The school can accommodate up to 200 girls ages 14 to 18.

Lebanon has provided shelter to 1.2 million of the four million refugees who fled war-torn Syria. Only about a fifth of the 500,000 Syrian school-age children in Lebanon receive formal education.

Although the country has allowed informal settlements on land leased by refugees, it said it can no longer keep up considering the unending exodus of refugees from Syria's four-year conflict.

One in every four people in Lebanon is a refugee, officials said.

"In Lebanon as well as in Jordan, an increasing number of refugees are being turned back at the border," Malala said. "This is inhuman and this is shameful."

The number of Syrian refugees in neighbouring countries is seen to grow to 4.27 million at this year's end.