Pope Francis attends his weekly general audience in the Paul VI Hall at the Vatican, Wednesday, Nov. 3, 2021. AP
The pontiff said he was following "with concern" the news from Ethiopia, "shaken by a conflict that has been going on for more than a year, and that has caused numerous victims and a grave humanitarian crisis".
"I renew my appeal that fraternal harmony and the peaceful path of dialogue may prevail," he told faithful gathered in St Peter's Square for the Angelus prayer.
Nine rebel groups said on Friday they would join forces in an alliance built around the Tigray People's Liberation Front (TPLF), which has been locked in a year-long war with the Ethiopian government.
The conflict has killed thousands and pushed many more into famine-like conditions.
Prime Minister Abiy Ahmed, winner of the 2019 Nobel Peace Prize, sent troops into Tigray in November last year to topple the TPLF, accusing them of attacking military bases.
The rebels have claimed several cities in recent weeks and have not ruled out marching on Addis Ababa.
The government, which has denied the capital is under threat, has nevertheless declared a state of emergency and local authorities have asked residents to organise to defend the city.
The United States, Saudi Arabia, Norway, Sweden, Denmark and other countries have all urged non-essential diplomats and their families to leave Ethiopia in the past week.