Nigeria's police have offered a $300,000 (£177,000) reward to anyone who can help locate and rescue more than 200 abducted schoolgirls.

They were kidnapped more than three weeks ago by Islamist Boko Haram militants from their boarding school in the north-eastern state of Borno.

Eleven other girls were taken on Sunday night after two villages were attacked.

Another militant raid on a town near Cameroon killed some 300 people on Monday, a senator has told the BBC.

Ahmed Zanna said the gunmen arrived in a convoy of vans in Gamboru Ngala during the town's busy market day.

They stole food and motorbikes, burned hundreds of cars and buildings during their rampage, the politician told the BBC's Hausa service.

It is the latest attack to be blamed on Boko Haram, whose leader admitted earlier this week that his fighters had abducted the girls in the middle of the night from their school in the town of Chibok on 14 April.

Abubakar Shekau threatened to "sell" the students, saying they should not have been in school in the first place, but rather should get married.

The group, whose name means "Western education is forbidden" in the local Hausa language, began its insurgency in 2009.

More than 1,500 have been killed in the violence and subsequent security crackdown this year alone.

'Heart-breaking'

A statement from the police said the 50m naira reward would be given to anyone who "volunteers credible information that will lead to the location and rescue of the female students".

Six telephone numbers are provided, calling on the general public to be "part of the solution to the present security challenge".

"The police high command also reassures all citizens that any information given would be treated anonymously and with utmost confidentiality," the statement said.

The abductions have prompted widespread criticism of the Nigerian government and demonstrations countrywide.

The BBC's Mansur Liman in the capital, Abuja, says many are questioning why it has taken so long for such a reward to be offered.

The girls are mostly aged between 16 and 18 and were taking their final year exams.

The governments of Chad and Cameroon have denied suggestions that the abducted girls may have already been smuggled over Nigeria's porous borders into their territory.

A team of US experts has been sent to Nigeria to help in the hunt.

On Tuesday, US President Barack Obama described the abductions as "heart-breaking" and "outrageous" and said he hoped the kidnapping might galvanise the international community to take action against Boko Haram.

UK Prime Minister David Cameron will be speaking by phone to Nigeria's President Goodluck Jonathan on Wednesday afternoon regarding the abductions.

Security has been tightened in Abuja as several African leaders and Chinese Premier Li Keqiang are attending the World Economic Forum for Africa in the city, following two recent attacks there blamed on the insurgents.