Eritrea on Saturday asked the UN Security Council to take "deterrent measures" after what it called Ethiopia's "declaration of war" against it in the conflict over the Tigray region.
The neighbouring countries have accused each other of starting a war over Tigray, where Ethiopia says Eritrea is backing rebels.
Rebels launched attacks against federal forces last month in the northern Ethiopian region. Eritrean forces have since invaded Ethiopian territory.
The Ethiopian army has pushed back the rebel Tigray People's Liberation Front (TPLF) and seized their capital, Mekelle.
It has also launched drone strikes against the Eritrean troops in the region.
"Eritrea requests the UN Security Council to forcefully condemn Ethiopia's deplorable declaration of war against Eritrea... and take the requisite deterrent measures," Eritrean Foreign Minister Osman Saleh said in a letter.
He said the Ethiopian action would cause "huge devastation to the peoples of both countries and destabilize the whole region".
Ethiopian Foreign Minister Gedion Timothewos on Friday accused Eritrea of "a blatant act of aggression that engages Ethiopia's inherent right to self-defence".
Eritrea gained independence from Ethiopia in 1993 and they fought a deadly border war from 1998 to 2000.
Relations improved after Ethiopian Prime Minister Abiy Ahmed came to power in 2018, but have soured again in recent years.
Landlocked Ethiopia has repeatedly insisted that it has a right to direct sea access, which Eritrea interprets as a threat to invade and seize its port at Assab.
TPLF fighters battled Ethiopian government forces, then backed by Eritrea, in a devastating 2020-2022 war.