The international community must not give up hope of finding the missing schoolgirls kidnapped by Boko Haram in Nigeria two years ago, one of the young women who managed to escape has said.

Speaking at a congressional hearing in Washington on Wednesday, Sa'a (not her real name) recalled the night of April 14, 2014 when Boko Haram militants overran her school in Chibok, Borno state, in northern Nigeria.

She said the jihadists shouted "Allahu Akbar" (God is Greatest) as they entered the school, forcing all the girls out of bed and into classrooms. "Next, they started burning everything – our clothes, our books, our classrooms – everything in our school," Sa'a said.

The militants then marched the girls to their trucks, and forced them in. "If we did not, they were going to shoot all of us," she said. Sa'a and a friend managed to jump out as they were driving through the forest. "I would rather die so my parents will see my body and bury it than to go with the Boko Haram."

Sa'a now lives and attends college in the US through the Education Must Continue Initiative; a charity run by and for victims of Boko Haram's insurgency. Around 3,000 displaced children and young people have returned to school through the scheme.

However, she remains troubled by the difficulties still experienced by millions of people in Nigeria.

A report released by Human Rights Watch (HRW) in April warned that one million children have little or no access to schools as a result of Boko Haram's attacks.

More than 910 schools have been targeted by the Islamist group, whose name means "Western (or non-Islamic) education is a sin". At least 611 teachers have been deliberately killed and another 19,000 forced to flee. At least 1,500 schools have closed.

In a video released in May 2014 Boko Haram leader, Abubakar Shekau, said women and girls would continue to be abducted to "turn them to the path of true Islam" and ensure they did not attend school.

The fighting has sparked a largely unreported refugee crisis with an estimated 2.2 million people, including 1.4 million children, displaced. Only around 10 per cent are in government-recognised refugee camps where there is some schooling. The other 90 per cent are living with friends and family members with little or no access to education.

HRW researcher Mausi Segun said: "In its brutal crusade against western-style education, Boko Haram is robbing an entire generation of children in northeast Nigeria of their education."

Sa'a wants to study science and medicine and use her knowledge to help her country, but doesn't know if it will ever be safe enough.

"Many live in fear every day. Their homes were burnt, so many people didn't have a place to sleep, food to eat or clothes to wear... The Nigerian government promised to rebuild the Chibok school, but it is still burned two years later," she said.

"Thanks to God, I am safely here in the US and doing well with my studies, but I worry about my family in Nigeria. People ask me if it will be safe for me to return to Nigeria. I ask, is it safe for anyone in Northern Nigeria?"

Sa'a recalled seeing a video released by Boko Haram on the two year anniversary of the Chibok kidnapping.

More than 200 girls taken that night are believed to remain in captivity, The footage, believed to have been filmed on Christmas Day last year, showed 15 girls dressed in black hijabs and talking to a cameraman off-screen.

It was the first possible sighting of the girls since Boko Haram released a video of them in May 2014.

"I am glad to see that some of them are alive," Sa'a said. "The moment I saw them and recognised their faces, I started crying, with tears of joy coming rolling down from my eyes, thanking God for their lives. Seeing them has given me more courage not to give up. Seeing them gives me the courage to tell the world today that we should not lose hope."

"I have had dreams," she continued. "With what I have been through, some of the dreams are scary. But now my dreams are good. I have a dream of a safe Nigeria; a Nigeria where girls can go to school without fear of being kidnapped; a Nigeria where girls like me are not made into suicide bombers and little boys are not routinely stolen and turned into terrorists; a Nigeria, where even if the worst happens and children are stolen, that every effort is made for their swift rescue; that those who can help will help; and that those who can speak will speak out for those who can't speak for themselves.

"I dream and I pray for freedom, safety, and peace to win in Nigeria."

Speaking ahead of the congressional hearing,  US Rep Chris Smith, chairman of the Africa and Global Human Rights Subcommittee, said Boko Haram was "now the world's most dangerous terrorist group".

Frank Wolf,  a former US Rep and distinguished senior fellow at the 21st Century Wilberforce Initiative, noted that according to the Global Terrorism Index 2015, Boko Haram killed more people than Islamic State in 2014, making them "the single most deadly terror organization in the world".

He warned that "the crisis plaguing Nigeria is multi-faceted, but one that must be addressed by the Nigerian government, our government, and the international community".

"The challenges that face Nigeria are great," Wolf added. "However, it is my firm belief that the United States and other Western nations have a vested interest in confronting one of the worst crises of our current day. Nigeria has been fractured and forgotten and it is my hope that this hearing may light the spark that is needed to elevate this crisis to the place it deserves."