Rebels from Ethiopia's Tigray region fired rockets at the capital of neighbouring Amhara region, Amhara authorities said, raising concern that a conflict between the insurgents and the central government could spill into a wider war.
Two weeks into the conflict, the United Nations said it was making plans for as many as 200,000 refugees fleeing into neighbouring Sudan.
Hundreds, perhaps thousands, of people have been killed and tens of thousands of refugees have already fled from fighting in Tigray as Prime Minister Abiy Ahmed tries to hold his ethnically diverse nation together.
"The illegal TPLF group have launched a rocket attack around 1.40 am in Bahir Dar," the Amhara government's communications office said, referring to the Tigray People's Liberation Front.
The rockets caused no damage, it said.
Bahir Dar, Amhara region's lakeside capital, is located hundreds of miles from the fighting in Tigray.
Tigrayan refugees have told Reuters the Amhara militiamen are fighting on the government side, and the two regions have a border dispute.
A local journalist and another resident in Bahir Dar said they heard two explosions and had been told by people in the area that at least one missile hit near the airport.
At the Um Rakuba settlement in Sudan, where aid workers are setting up a camp in the scrubland for 10,000 mainly Tigrayan refugees, those who arrived queued up for cornflour porridge and put up makeshift shelters.
Yohannes Gor, 28, arrived by foot after fleeing the Ethiopian town of Humera, close to the border, 10 days ago with only the clothes he was wearing.
"I live under this tree and sleep on the dirt," he said. "I lost all trace of my family and I don't know what happened to them."
Aid agencies fear a humanitarian emergency in Tigray, where hundreds of thousands of people depended on relief aid even before the conflict. Refugees have crowded into boats to cross a river to Sudan, overwhelming aid groups on the other side.
Axel Bisschop of the UN. refugee agency UNHCR told a briefing in Geneva that 31,000 refugees had already reached Sudan, surpassing a contingency plan for 20,000. "The new planning figure is around 200,000."
UNICEF's Sudan-based representative Abdullah Fadil worried about the impact on Sudan, already hosting 1 million refugees from other African wars.
"Our serious concern is if we do not act quickly with the resources needed ... this could unravel not only Ethiopia but also Sudan," he told the same briefing.
Ethiopia, a federation of 10 ethnic regions, was dominated for decades by Tigrayans in a TPLF-led ruling coalition, until Abiy, who is of Amhara and Oromo descent, took power two years ago. He says he aims to share authority more fairly. The TPLF accuses him of pursuing a vendetta against former officials.
The conflict erupted two weeks ago after what the government called a TPLF attack on army forces stationed in the region.
Yesterday, Tigray forces accused the government of bombing a university in the Tigray capital Mekelle. There was no immediate response from the government, although officials have said they are attacking only military targets.
It has been impossible to verify assertions on all sides because telephone lines and internet links to Tigray have been severed since the conflict began.
Ethiopia says it is closing in on Mekelle, which the rebels have said they will defend.
A week ago, Tigray forces fired rockets at two airports in Amhara. They have also fired rockets into the neighbouring nation of Eritrea, which has a long-running enmity with the TPLF leadership.
Eritrea made peace with Ethiopia in 2018, earning Abiy the Nobel Peace Prize.
Reports of ethnically motivated killings have emerged. Rights group Amnesty International documented a mass killing of civilians, many of whom appeared to be Amhara, by what it says were Tigrayan forces on November 9-10, which the Tigray authorities denied.
Refugees fleeing into Sudan have said they were targeted for being Tigrayan.
Since taking power, Abiy has freed political prisoners and loosened what was once one of Africa's most repressive political and economic systems.
But the new freedoms have been accompanied by violence as regional bosses vie for power, money and land.