The federal government in Ethiopia has vowed to continue a military offensive in the northern Tigray region despite international calls for restraint. The row between the Tigray People's Liberation Front (TPLF), once a dominant member of Ethiopia's ruling coalition, and Prime Minister Abiy Ahmed's government has been simmering for months, and came to the fore after a regional election was held defiance of the federal government. Prime Minister Abiy ordered the military to mount an offensive against Tigray, after he said an attack on a federal army base that resulted in "many martyrs, injuries and property damage". He blamed the region's ruling party, the Tigray People's Liberation Front (TPLF). What led to the tension? September's election in Tigray, which the federal government had postponed nationwide because of coronavirus, is widely considered to be the cause of the recent rapid deterioration. But the tension has been building for longer. The TPLF, which was the dominant political party in Ethiopia for decades, has been feuding with Mr Abiy's government since shortly after he came to power in 2018. Elected as a "reformist leader", the prime minister accused officials in previous governments of corruption and human rights abuses, and removed key TPLF figures from the central government. This included the former intelligence chief and senior TPLF official, Getachew Asefa, who evaded arrest and fled to Tigray, where he remains as a fugitive. Mr Abiy's decision last year to merge the ethnically based parties that formed the governing EPRDF coalition and set up the Prosperity Party (PP) added fuel to the tensions. The TPLF opposed the decision, saying it would divide the country, and refused to join the PP. Earlier this year, the rift grew even wider after the federal government postponed nationwide elections. Tigray's decision to hold its own vote in September was an unprecedented act of defiance against the federal government. The federal parliament termed the process "illegal". Since then, both governments have designated each other as "illegitimate and unconstitutional". The TPLF had previously made veiled threats of secession, citing an article in the federal constitution which allows the "unconditional right to self-determination, including the right to secession". "We will never back down for anyone who is intending to suppress our hard-won right to self-determination and self-rule," the region's leader, Debretsion Gebremichael said in August. Early in October, the federal government decided to cut ties with the Tigray region and the upper house of parliament voted to suspend budget aid to Tigray.