ADDIS ABABA (Reuters) - Ethiopia’s prime minister stepped up a military offensive in the northern region of Tigray on Sunday with air strikes as part of what he called a “law enforcement operation”, increasing fears of outright civil war in Africa’s second-most populous country.Prime Minister Abiy Ahmed has defied calls from the United Nations and allies in the region to negotiate with leaders in Tigray, home of the ethnic group that dominated the federal government before he took power in 2018. Abiy last week launched a military campaign in the province, saying forces loyal to leaders there had attacked a military base and attempted to steal equipment. Abiy accuses the leaders of Tigray of undermining his democratic reforms. Government fighter jets have since been bombing targets in the region, which borders Sudan and Eritrea. Aid workers on Sunday reported heavy fighting in several parts of the region, with at least six dead and dozens wounded. Also on Sunday, Abiy named a new chief of the army, a new intelligence chief and a new federal police commissioner and foreign minister, changes that analysts said brought close allies into top posts as the conflict escalates. The premier won last year’s Nobel Peace Prize for making peace with neighbouring Eritrea and for introducing democratic reforms in one of Africa’s most repressive countries.