KADUNA, Nigeria, Aug 16 (Reuters) - Gunmen abducted 15 students and four staff members from an agricultural college in the northwestern Nigerian state of Zamfara, police said on Monday, the latest in a spate of mass kidnappings across Africa"s most populous country. During the attack late on Sunday night, the gunmen exchanged fire with a security team protecting the college, resulting in the deaths of one police officer and two guards, according to Mohammed Shehu, a spokesperson for Zamfara State police. He said officers were combing the area surrounding the town of Bakura, where the college is located, in search of the victims, and three members of staff had been found and rescued. Abductions for ransom, particularly at schools and colleges, have increased in northwestern Nigeria over the past eight months, partly driven by a sharp rise in poverty levels linked to the economic shock caused by the COVID-19 pandemic in 2020. Over 1,000 children and students have been taken from their places of education since December. Some have been released following ransom payments but many are still missing. The abductions are one aspect of a broader trend of rising insecurity across Nigeria, with armed robberies and conflict between communities also contributing to a heavy death toll. Reporting by Garba Muhammad, writing by Estelle Shirbon, editing by Timothy Heritage Our Standards: The Thomson Reuters Trust Principles.