ADDIS ABABA/NAIROBI (Reuters) - Two different groups fighting Ethiopia's central government said they had seized control of towns on Sunday as the prime minister appealed for citizens to take up arms. The spreading conflict threatens to further destabilise Africa's second most populous nation, once considered a stable Western ally in a volatile region. Prime Minister Abiy Ahmed urged citizens to join the fight against the Tigray People's Liberation Front (TPLF), the party in control of the rebellious northern region of Tigray, after Tigrayan forces said they took another town on a highway link...