Cameroon on Wednesday blamed separatists over the killing of a US missionary, but Washington said its citizen was caught in crossfire and urged dialogue to end the African country's conflict.Charles Wesco, 44, was travelling with his wife, son and a driver in the restive English-speaking northwest region when militants attacked security forces and a university campus, Defence Minister Joseph Beti Assomo said.In a statement read on army radio, Beti Assomo said the American came under fire in an ambush by "a group of armed terrorists" in the town of Bambui and died later in a hospital in the regional capital Bamenda, 14km away.The minister said that security forces then pursued the militants, killing four of them, with several others wounded including a soldier and a student.His comments came amid speculation by separatist supporters on social media in the English-speaking regions - the site of an uprising by armed secessionists - that the Cameroon army was behind the killing. But in Washington, State Department spokesperson Robert Palladino did not blame either side and said Cameroon had promised the United States a "full investigation" into Wesco's death."We've been in touch with senior Cameroonian government security officials and we believe the victim was caught in the crossfire," Palladino told reporters.Palladino urged the two sides to start "an immediate and broad-based dialogue without preconditions" to bring peace and resolve grievances."Both separatist fighters and government security forces have used violence against innocent civilians," Palladino said."We call on both sides to view the tragic death of this American citizen as an opportunity to put an immediate stop to violence and to allow unhindered access for humanitarian aid workers and health care providers in the northwest and southwest regions," he said.Grievances by English speakers English-speaking separatists argue that the French-language education system penalises anglophone students in the largely francophone central African nation.The unrest has claimed the lives of more than 400 civilians and an unknown number of separatists in the year to September, according to the International Crisis Group think-tank.* Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and FacebookOn Thursday, gunmen killed a linguistics professor in the same area.University staff said the murder of the academic, who was also in charge of administration, bore the hallmarks of the "Amba boys," or separatists fighting for an independent English-speaking state called Ambazonia.The government has refused to engage in dialogue with anglophone separatists it brands "terrorists" and has sent forces into the area to restore order.More than 300 000 people have fled the violence, some to neighbouring Nigeria.The voter turnout during October's presidential election was very low in both anglophone regions, although Paul Biya won more than two-thirds of the votes cast, according to official results.Biya, 85, has been in power since 1982 and was re-elected for a seventh term with 71% of the votes nationwide.