ABUJA, Nigeria — The United States is preparing to launch a "major" border security program to help Nigeria and its neighbors combat the increasing number and scope of attacks by Islamic extremists, a senior U.S. official for Africa said Thursday.Nigerian insurgents have begun attacking villages in neighboring Cameroon and have been seizing land in northeast Nigeria where they proclaimed an Islamic caliphate.Assistant Secretary of State Linda Thomas-Greenfield told a meeting of U.S. and Nigerian officials in Abuja, the Nigerian capital, that "Despite our collective efforts, the situation on the ground is worsening."The frequency and scope of Boko Haram's terror attacks have grown more acute and constitute a serious threat to this country's overall security," she said. "This is a sober reality check for all of us. We are past time for denial and pride."The government denied that Boko Haram this week overran Bama, the second largest city in Borno state, but Thomas-Greenfield said, "We are very troubled by the apparent capture of Bama and the prospects for an attack on and in Maiduguri."And the U.N. Office for the Coordination of Humanitarian Affairs said it is "extremely concerned about the further deterioration of the security situation following the recent attack by Boko Haram on Bama town."Bama, a city of about 200,000 people is just 75 kilometers (45 miles) from Maiduguri, the Borno state capital that is the headquarters of the military campaign against Boko Haram.A senior Borno state security officer said a bombing raid Wednesday destroyed a Boko Haram camp outside Bama. The officer said two Air Force jets spotted the insurgents, who apparently were gathering for a meeting. He spoke to The Associated Press on condition of anonymity because he is not authorized to give information to journalists.The officer and a member of the Nigerian Vigilante Group of civilians fighting Boko Haram both said some of the thousands of Bama residents who fled were returning home. There was no way to independently verify the confusing and conflicting information about Bama.OCHA, the U.N. agency, said at least 26,000 people have been displaced in Bama.A Westerner working on the Cameroon side of the border said more than 100 Nigerian soldiers fled Bama and crossed into Cameroon on Tuesday. Last week, nearly 500 Nigerian soldiers fled into Cameroon from another border town that was under attack.The United States has flown unarmed drones to spy in parts of northeast Nigeria in a joint effort to try to save more than 200 schoolgirls kidnapped by the extremists in April."The Chibok schoolgirls and others remain hostages, enduring horrible and tragic suffering," Thomas-Greenfield said.She added that Cameroon's military is increasingly forced to fight Boko Haram within Cameroon, and that the insurgents flee back into Nigeria without fear.The U.N. refugee agency said insurgents had begun attacking villages in northern Cameroon last week, and spokeswoman Helene Caux said Thursday that Cameroonian authorities say some 5,000 Cameroonians have fled the assaults. UNHCR says 645,000 Nigerians are displaced inside their country by the insurgency while tens of thousands have fled into Cameroon and Niger.Nigeria's home-grown insurgency grew out of a cult that preached against endemic corruption. Now Boko Haram — the name means Western education is sinful — has declared an Islamic caliphate in seized land in the extreme east of Borno state and says it wants to create an Islamic state in all of Nigeria, Africa's most populous nation of about 170 million people divided almost equally between Muslims and Christians.Nigeria's Foreign Minister Aminu Wali on Wednesday told a regional security meeting of foreign ministers that Nigeria's government remains puzzled about funding and arms supply of Boko Haram."Who are the sponsors of Boko Haram terrorist campaigns? Who are those funding the insurgency? Where are the sources of the sophisticated arms and ammunition being used by the terrorists? Who are those seeking to re-define the territory of Nigeria and Africa in the 21st century?" he asked.--Faul reported from Johannesburg.