ADDIS ABABA/NAIROBI (Reuters) - Ethiopia’s military is fighting battle-hardened troops in the northern Tigray region, threatening stability around the Horn of Africa.The Ethiopian National Defence Force (ENDF) has around 140,000 active personnel, the vast majority of them in the army, according to the Janes security data group. Its troops have been tested by Islamist militants in Somalia and rebel groups in Ethiopia’s border regions, as well as the two-decade border war then standoff with Eritrea that only ended in 2018. Its air force gives it dominance in the skies over Tigray. According to Janes, it has 15 Sukhoi Su-27SK and eight MiG-23ML fighter jets, around 20 Mi-24 and Mi-35 helicopter gunships and a range of air defence and missile systems, as well as scores of Russian T-55 and T-72 tanks. A senior diplomat working on the Ethiopia crisis said Prime Minister Abiy Ahmed had increasingly fallen back on support from forces from Tigray’s southern neighbour Amhara in ground fighting - raising the risk of ethnic violence.