US urges Eritrea to pull out of Tigray after 'credible' reports
"We are aware of credible reports of Eritrean military involvement in Tigray and view this as a grave development," a State Department spokesperson said.
"We urge that any such troops be withdrawn immediately."
Eritrea has come under rocket fire at least three times since Ethiopian Prime Minister Abiy Ahmed launched an offensive on November 4 in its dissident-led northern region.
The rocket attacks were seen as a way to internationalize the conflict by drawing in Eritrea's authoritarian regime, which has historic tensions with Ethiopia but has found common cause with Abiy against the Tigray People's Liberation Front (TPLF).
The United States earlier applauded Eritrea for showing restraint after the rocket attacks and said it had been in contact with the regime, one of the world's most secretive.
Abiy ordered the offensive following alleged attacks in Tigray on federal military camps.
Abiy declared victory on November 28 but access remains tightly controled and the TPLF - which had dominated Ethiopian politics for nearly three decades until 2018 - has vowed to keep up its fight.
The State Department also voiced alarm over "reports of human rights violations and abuses in the region."
"We and other international partners continue to urge an independent investigation of the reports and accountability for those found responsible," the spokesperson said.
"We continue to urge all parties to restore peace, protect civilians - including refugees - and allow unhindered humanitarian access in Tigray."
The United States is allied with Ethiopia but has increasingly voiced alarm over the humanitarian consequences of the Tigray offensive, including the flight of refugees to neighboring Sudan, whose new civilian-backed government is trying to turn the page on decades of conflict.