Malala Yousafzai, the youngest winner of the Nobel Peace Prize, celebrated her 18th birthday in Lebanon on Sunday by opening a school for Syrian refugee girls and called on world leaders to invest in "books not bullets".

Malala became a symbol of defiance after she was shot on a school bus in Pakistan 2012 by the Taliban for advocating girls' rights to education. She continued campaigning and won the Nobel in 2014.

"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 told Reuters in a schoolroom decorated with drawings of butterflies.

The Malala Fund, a non-profit organization that supports local education projects, paid for the school in the Bekaa Valley, close to the Syrian border. It can welcome up to 200 girls aged 14 to 18.

"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," Malala said in a speech.

Lebanon is home to 1.2 million of the 4 million refugees that have fled Syria's war to neighboring countries. There are about 500,000 Syrian school-age children in Lebanon, but only a fifth are in formal education.

Lebanon, which allows informal settlements on land rented by refugees, says it can no longer cope with the influx from Syria's four-year conflict. One in four living in Lebanon is a refugee.

The UN says the number of Syrian refugees in neighboring countries is expected to reach 4.27 million by the end of the year.

"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."

Her father Ziauddin said he was proud she was carrying on her activism into adulthood.

"This is the mission we have taken for the last 8-9 years. A small moment for the education of girls in Swat Valley: it is spreading now all over the world," he said.

Malala was feted with songs and a birthday cake. Moved to tears by the girls, she was modest when asked for advice.

"They are amazing, I don't think they need any message, I don't think they need any other advice because they know that education is very important for them."