Malala Yousafzai has called on Nigeria to intensify efforts to free 219 schoolgirls who were abducted by Islamist militants six months ago.

The Nobel Peace Prize winner said campaigners needed to raise their voices "louder than ever" to demand the freedom of the girls.

The Boko Haram group sparked global outrage when it seized the girls.

Foreign governments including the US and China, have sent experts to Nigeria to help track them down.

Boko Haram fighters abducted the girls during a raid on their boarding school in Chibok town in north-eastern Nigeria in April.

Malala said in a statement that the schoolgirls needed to be reunited with their families and receive a quality and safe education.

"I urge the Nigerian government and the international community to redouble their efforts to bring a quick and peaceful conclusion to this crisis," Malala said.

Critics accuse government of not doing enough to secure the release of the girls - a charge ministers deny.

A mother's pain

"We are in a desperate situation. Sometimes, when we go to the farm and remember what has happened we just start crying and can't work," Hannatu Dauda told the BBC.

She says she last heard from her abducted daughter, Saratu, when Boko Haram raided her boarding school in Chibok in April.

"When she called, we were all lying in the compound. She said: 'Some people have come to take us and they have rounded us up. Please pray for us'. And then her father and the rest of us kept praying," Mrs Dauda recalls.

"After some minutes she called again to say: 'Daddy they have taken all of us from school. We have been loaded onto a truck and we don't know where they are taking us to. Please tell my mummy to forgive me until we meet again'," she adds.

Will Nigerian schoolgirls ever be freed?

Activists earlier tried to march to the official residence of President Goodluck Jonathan in Abuja.

But Maureen Kabrik, an organiser of the Bring Back Our Girls campaign in Nigeria, said an "army of policemen" halted the protest.

"We're here. We're a civil group. We're not a disobedient group," she told BBC Focus on Africa.

"Yet, they stopped us with heavy arms."

The police have not commented on why they blocked the march.

Malala, a 17-year-old Pakistani campaigner for girls' education, met Mr Jonathan in July to discuss the abductions.

She won the Nobel Peace Prize earlier this month, making her the youngest ever recipient of the award.