Eritrean troops have launched a full-scale offensive in Ethiopia's Tigray region, a spokesman for the Tigrayan rebel group, the TPLF, has said.
A US envoy said the US was aware of Eritrean troops crossing into Tigray, and condemned it.
The Eritrean and Ethiopian governments have not yet commented on the claims.
If Eritrea's offensive against the rebels is confirmed, it will mark an escalation following the collapse of a five-month truce last month.
Conflict first broke out in November 2020 following a massive fall-out between Ethiopia's Prime Minister Abiy Ahmed and the Tigray People's Liberation Front (TPLF), which controls the region.
Eritrea intervened on the side of the Ethiopian military, but it reportedly withdrew most of its troops last year.
On Tuesday, TPLF spokesman Getachew Reda said heavy fighting was taking place in several areas along the Tigray-Eritrea border.
"Eritrea is deploying its entire army as well as reservists. Our forces are heroically defending their positions," he said in a tweet, adding that Ethiopian troops were also taking part in the offensive.
The US envoy to the region, Mike Hammer, said the US had been tracking Eritrean troop movements across the border.
"They're extremely concerning and we condemn it. The presence of Eritrean troops in Ethiopia only serves to complicate matters, and inflame an already tragic situation," he said.
Thousands of people have been killed and millions have been displaced in the conflict.
Access to Tigray is heavily restricted and it is difficult to get independent confirmation of the fighting.
Two aid workers told the Reuters news agency there was intense fighting on the border but could not confirm if Eritrean troops had crossed into Tigray.
Last week, BBC Tigrinya reported that Eritrea was mobilising military reservists to bolster its army.
Mobilisation notices were distributed in the capital Asmara, the second-largest city, Keren, the western town of Tessenai and other areas, witnesses said.
However, Eritrea's Information Minister Yemane Gebremeskel said that a "tiny number" of reservists had been called up, denying that the entire population had been mobilised.