Ethiopia's Amhara region on Sunday called on all armed residents to mobilise for battle against rebels from conflict-hit Tigray, calling it a "survival campaign", state media reported.
Amhara borders Tigray to the south, and the two regions are embroiled in a decades-old land dispute that has become central to the eight-month-old war in Tigray.
Sunday's statement from Amhara regional president Agegnehu Teshager echoes a call made Friday by the president of Ethiopia's Afar region just east of Tigray.
Together the two statements highlight the potential for the Tigray war, which Prime Minister Abiy Ahmed declared all but over in late November, to draw in the rest of the country.
"Starting from tomorrow (Monday), I call on all people of age who are armed either at governmental or private level to mobilise for a survival campaign," Agegnehu said.
"We have called on the general public to stand on our side. Now, the public is on our side in every aspect," he added.
"The support we are receiving from civil servants in the region is overwhelming. We are proud of that."
Abiy, winner of the 2019 Nobel Peace Prize, sent troops into Tigray last November to oust the region's ruling party, the Tigray People's Liberation Front (TPLF), a move he said was in response to TPLF attacks on federal army camps.
Fighting dragged on for months before the war took a stunning turn in late June, with pro-TPLF fighters reclaiming the Tigray capital Mekele and Abiy declaring a unilateral ceasefire.
Yet clashes have continued and officials from six regions and the city of Dire Dawa have since said they would send troops to back up government forces.
At least 20 civilians were killed and tens of thousands displaced in fighting in Afar last week, an official said.
Rebel spokesman Getachew Reda has vowed to "liberate every square inch of Tigray", including its western and southern portions, disputed territories that have been occupied by Amhara forces since the beginning of the war.
A communications blackout in Tigray makes it difficult to confirm who holds which territory.
The war has already killed thousands and, according to the United Nations, pushed hundreds of thousands into famine.