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Ben Curtis, Associated Press
Opposition candidate Gen. Muhammadu Buhari, right, looks across as he and Nigeria's President Goodluck Jonathan, left, prepare to sign a renewal of their pledge to hold peaceful "free, fair, and credible" elections, at a hotel in the capital Abuja, Nigeria Thursday, March 26, 2015. Nigerians are due to go to the polls to vote in presidential elections on Saturday.

ABUJA, Nigeria — The two leading candidates in this weekend's presidential elections pledged Thursday to accept the outcome as a new report showed that Boko Haram Islamic extremists, who are threatening to disrupt the vote, have killed more than 1,000 civilians this year.

The violence last month forced postponement of presidential elections that are now scheduled for Saturday. Boko Haram is also using an unknown number of civilians as human shields as its fighters flee a multinational military force in northeast Nigeria, a government spokesman, Mike Omeri, said Wednesday.

The main contenders, President Goodluck Jonathan and former military dictator Maj. Gen. Muhammadu Buhari, on Thursday re-committed themselves to a peaceful outcome. The two met and signed a peace pledge in Abuja, the capital, similar to one they made in January.

Buhari told reporters afterward that it does not mean a blanket acceptance of results. "How can we accept? ... I said under free, fair and credible elections ... That is the caveat."

Reports indicate that Boko Haram has taken to slaughtering and kidnapping civilians as it retreats before the regional force led by troops from Nigeria and Chad.

"Each week that passes we learn of more brutal Boko Haram abuses against civilians," Nigeria researcher Mausi Segun is quoted as saying in a Human Rights Watch report issued on Thursday.

The New-York based group also charged Nigerian security forces torched a northeastern village in December and killed at least five civilians there. It said the military has promised to investigate the latest allegation of human rights abuses."

The report said "horrific levels of violence" have left people desperate despite the multinational offensive.

"Boko Haram fighters have deliberately attacked villages and committed mass killings and abductions," Human Rights Watch said, adding that Boko Haram also kidnapped hundreds of women and girls, many of whom were subjected to forced religious conversion, forced marriage, rape, and other abuse. Scores of young men and boys were forced to join Boko Haram's ranks or face death, the report said.

On Wednesday night the government closed all Nigeria's land and sea borders until midnight Saturday as a security precaution before the election.