"We've been tracking Eritrean troop movements across the border. They're extremely concerning and we condemn it," Washington's special envoy to the region, Mike Hammer, told reporters on Tuesday.

"The presence of Eritrean troops in Ethiopia only serves to complicate matters, and inflame an already tragic situation."

On Tuesday, authorities in Tigray accused Eritrean forces of launching a "full scale offensive" across northern Ethiopia, where heavy combat has been reported on multiple fronts in recent weeks.

AFP was not able to independently verify the claims. Access to northern Ethiopia is severely restricted and Tigray has been under a communications blackout for more than a year.

Eritrean soldiers have crossed the border to back Ethiopian troops since fighting reignited in August between government forces and rebels from the Tigray region, ending a five-month truce between the warring sides.

The latest fighting comes as diplomatic efforts intensify to try and peacefully resolve the nearly two-year war in Africa's second-most populous country.

Tigrayan authorities announced this month they were ready to participate in talks mediated by the African Union, removing an obstacle to negotiations with Prime Minister Abiy Ahmed's government in Addis Ababa.

But fighting between government forces and their allies and rebels led by the Tigray People's Liberation Front (TPLF) has only escalated.

Hammer, who just returned from a 11-day trip to Ethiopia, said the United States took both sides "at their word when they say they are committed to trying to find a peaceful resolution".

"But again, having foreign actors become involved only serves to exacerbate the crisis," he said.

Eritrean troops supported Ethiopian forces in the early stages of the war which erupted in November 2020 when Abiy sent soldiers into Tigray to unseat the TPLF, accusing the group of attacking federal army camps.

On Tuesday, TPLF spokesman Getachew Reda said Eritrea had deployed "its entire army as well as reservists" in a fresh push across northern Ethiopia alongside national and regional troops.

His statement followed a general call for mobilization by Eritrean authorities issued last week.

The war has claimed untold lives and spurred a humanitarian crisis, and all sides to the conflict have been accused of grave abuses against civilians.

UN investigators said Monday they believed Ethiopia's government was behind ongoing crimes against humanity in Tigray, and warned the resumption of the conflict there increased the risk of "further atrocity crimes".