The US blames Eritrea for playing a central role in the resumption of conflict in Ethiopia's Tigray region and called for Eritreans to "withdraw to their borders immediately".
Between March and August, there was a mini truce in Tigray as the Ethiopian government and transitional government of Tigray agreed to work towards an African Union-led peace deal to end the war that began in November 2020.
During that period, relief aid was allowed into Tigray and there were plans under way to resume normal life with the restoration of basic economic and social services.
But before much could be achieved, fighting broke out in August.
Last week, Getachew Reda, the former communication affairs minister of Ethiopia and now a spokesperson for the Tigray People's Liberation Front, tweeted Eritrea had re-joined as allies of the Ethiopian government in launching attacks.
"Eritrea is deploying its entire army as well as reservists. Our forces are heroically defending their positions," he tweeted.
Responding to journalists at a virtual press conference, Ned Price, the spokesperson for the US State Department, said the US had called on Eritrea to withdraw to its borders.
"We've also been very clear with Eritrea and Eritrean authorities that they must withdraw to their borders immediately and for Eritrea and others to cease fuelling the conflict. We're deeply concerned by the human rights abuses that this conflict has brought about."
Price added the only way back to safety was an AU-led peace initiative that America was in support of.
"We've called on the government of Ethiopia and the Tigray regional authorities to immediately halt their military offensives and to pursue a negotiated settlement through peace talks under the auspices of the African Union.
"We have worked very closely with the African Union, with other partners on the continent to engage in that process of diplomacy," he said.
Eritrea's interest in Ethiopia
Eritrea and Ethiopia shared a border that covered 1 051km through remote, desert areas, according to Sovereign Limits - a source of land and maritime international boundaries.
Eritrea, a former Italian colony, became part of Ethiopia after World War II but broke away in 1961 after an armed conflict.
It was recognised as an independent country by Ethiopia in 1993.
This is similar to what Tigray wants from Ethiopia.
Eritrea and Ethiopia would continue to have border disputes until 2018 when both agreed to accept Eritrean–Ethiopian Boundary Commission-suggested mappings.
Since then, relations have been good between Eritrea and Ethiopia.
Thus, some critics said Eritrea's fighting as allies with Ethiopia in Tigray was a diplomatic offensive to maintain good relations.