LAGOS, Nigeria (AP) -- Fighting between soldiers and Islamic extremists in northeast Nigeria killed at least 187 people, the worst single incident of violence in the region since an insurgency there began three years ago, an aid agency said Monday.
Nigeria's military blocked access for relief officials to enter the town of Baga, which sits along the shores of Lake Chad in the nation's far northeast, said Nwakpa O. Nwakpa, a Red Cross spokesman. Another 77 people are receiving medical care there in the ruins of a town where some 300 homes burned down, he said. Local residents blamed angry soldiers for burning down neighborhoods where they knew civilians were hiding.
"Our volunteers are on standby," Nwakpa said. "We are yet to be provided clearance."
The fighting in Baga began Friday and lasted for hours, sending people fleeing into the arid scrublands surrounding the community. By the time Borno state officials could reach the city Sunday, a local government official said at least 185 people were killed, something not disputed by a brigadier general who attended the visit.
Officials could not offer a breakdown of civilian casualties versus those of soldiers and extremist fighters. Many of the bodies had been burned beyond recognition in fires that razed whole sections of the town, residents said. Those killed were buried as soon as possible, following local Muslim tradition.
U.N. Secretary-General Ban Ki-moon expressed shock and sadness at the high civilian casualty toll and large number of homes destroyed and called on extremist groups to cease their attacks, U.N. spokesman Martin Nesirky said.
"The secretary-general reiterates his firm conviction that no objective sought can justify this resort to violence," Nesirky said. "He underscores the need for all concerned to fully respect human rights and safeguard the lives of civilians."
Nigerian President Goodluck Jonathan, who appeared at an event Monday on the nation's power supply at the Aso Rock presidential villa in the country's capital, did not comment on the killings there. A few hours after Ban's statement, Jonathan's office issued a release saying he had ordered "a full-scale investigation into reports of high civilian casualties."
The "administration will continue to do everything possible to avoid the killing or injuring of innocent bystanders in security operations against terrorists and insurgents," the statement read. "Rules of engagement for the military and security agencies are already in place for this purpose and the investigation ordered by President Jonathan into the incident in Baga is to amongst other things, determine whether or not these rules were fully complied with."
The statement also said "casualty figures being reported by the foreign media may be grossly exaggerated." The statement did not offer any tolls for the injured or dead, nor did it explain what military officials already had told the presidency about the fighting.
Members of the Islamic extremist network Boko Haram used heavy machine guns and rocket-propelled grenades in the assault Friday, which Brig. Gen. Austin Edokpaye said began after soldiers surrounded a mosque they believed was housing Boko Haram members. Extremists earlier had killed a military officer, officials said.
The military said extremists used civilians as human shields during the fighting -- implying that soldiers opened fire in neighborhoods where they knew civilians lived.
However, local residents who spoke to an Associated Press journalist who accompanied the state officials said soldiers purposefully set the fires during the attack. Violence by security forces in the northeast targeting civilians has been widely documented by journalists and human rights activists. A similar raid in Maiduguri, Borno state's capital, in October after extremists killed a military officer saw soldiers kill at least 30 civilians and set fires across a neighborhood.
Eric Guttschuss, a researcher with Human Rights Watch who studies Nigeria, said his organization continued to study what happened, though it already had deep concerns about the allegations surrounding the soldiers' conduct.
"We are investigating this extremely serious incident," he said. "In the past, Nigeria has simply denied or tried to cover up security force abuses."
The Islamic insurgency in Nigeria grew out of a 2009 riot led by Boko Haram members in Maiduguri that ended in a military and police crackdown that killed some 700 people. The group's leader died in police custody in an apparent execution. From 2010 on, Islamic extremists have engaged in hit-and-run shootings and suicide bombings, attacks that have killed at least 1,548 people before Friday's attack, according to an AP count.
Boko Haram, which means "Western education is sacrilege" in the Hausa language of Nigeria's north, has said it wants its imprisoned members freed and Nigeria to adopt strict Shariah law across the multiethnic nation of more than 160 million people. While Jonathan has started a committee to look at the idea of offering an amnesty deal to extremist fighters, Boko Haram's leader Abubakar Shekau has dismissed the idea out of hand in messages.