ABUJA, Nigeria — Hundreds of civilians, including many children, have been abducted and are being used as human shields by Boko Haram extremists, a top Nigerian official confirmed Wednesday.

The news of the mass kidnappings comes as Nigeria prepares for crucial elections on Saturday.

Several hundred people were taken captive by the Islamic militants as they retreated earlier this month from Damasak in northeastern Nigeria, Mike Omeri, the Nigerian spokesman for the fight against Boko Haram, told The Associated Press Wednesday. He said he could not specify how many were taken captive but local reports say as many as 500 people were seized.

When troops from Chad and Niger advanced toward Damasak, Boko Haram began taking captives, said Omeri.

"Boko Haram ... rushed to primary schools they took children and adults that they are using as shields to protect themselves from the menacing advance of troops," said Omeri. "They are being used as shields by Boko Haram."

Damasak, near the border with Niger, was recaptured from Boko Haram on March 16. The kidnappings of civilians has only been confirmed now.

The soldiers who recaptured Damasak found the town largely deserted. Damasak had been held for months by Boko Haram, who used the trading town as an administrative center.

The troops from Chad and Niger who now hold Damasak have discovered evidence of a mass grave, Chad's ambassador to the U.N. Mahamat Zene Cherif confirmed Wednesday.

Almost a year ago some 276 girls were kidnapped before dawn from a government boarding school in Chibok. Dozens escaped in the first couple of days, but 219 remain missing. The case of the missing schoolgirls has gained widespread international attention and spawned the #BringBackOurGirls campaign on social media.

Nigeria's battle against the Islamic extremists is a major issue for the elections to be held Saturday. The 6-year-old Islamic insurgency has killed thousands, including an estimated 10,000 last year. Boko Haram has vowed to violently disrupt the elections.

International assistance is needed for the thousands of Nigerian refugees who have fled the violence, said the head of the U.N. refugee agency.

Some 74,000 Nigerians have fled to neighboring Cameroon, according to the agency. Over 100,000 more have flooded into Chad and Niger. Troops from the three countries are now helping Nigeria to combat the militants and win back Nigerian towns.

The refugee agency will funnel more resources to Cameroon, said U.N. High Commissioner for Refugees Antonio Guterres on Wednesday while visiting Maroua, the capital of Cameroon's Far North region. He stressed that additional assistance is needed.

"Cameroon is today not only a very important protection space for refugees, but it is in the first line of defense of the international community," he said.

The U.N. agency says the Nigerian crisis is one of the most underfunded in the world. In February, the agency asked for $71 million to assist displaced people in Nigeria and neighboring countries; already that figure appears to be too low, it said this week. Thus far, it has received only $6.8 million in donations, he said.

Moki contributed to this report from Maroua, Cameroon. AP writer Cara Anna contributed from the United Nations.