Ethiopia has accused Eritrea of preparing to wage war against it in collusion with an opposition grouping, in the latest sign of escalating tensions between the two neighbours over control of the Red Sea.

Ethiopia's accusation is contained in a letter sent by its Foreign Minister Gedion Timothewos to UN chief Antonio Guterres, warning that the collusion had become "more evident over the past few months".

Eritrea has not yet commented on the letter, but its relationship with Ethiopia has become increasingly strained in recent months.

Ethiopia has been rallying support to regain access to the Red Sea, causing alarm in Eritrea as it took control of the coastline at independence in 1993.

If Gedion's allegations are true, it means that Eritrea is waging a proxy war against Ethiopian Prime Minister Abiy Ahmed's government, to prevent it from launching a military incursion into Eritrea to seize ports along the Red Sea.

Ethiopia and Eritrea fought for control of the border town of Badme from 1998 to 2000, causing the deaths of tens of thousands of people.

After Abiy took office in 2018, he ended tensions by building a strong alliance with Eritrea's President Isaias Afwerki. However, relations between them have since soured, with access to the Red Sea becoming a major flashpoint.

In the letter seen by AFP news agency, Gedion alleged that Eritrea and a hardline faction of the Tigray People's Liberation Front (TPLF) were "funding, mobilizing and directing armed groups" in Ethiopia's Amhara region, where militiamen known as Fano have been battling the federal government.

"The hardliner faction of the TPLF and the Eritrean government are actively preparing to wage war against Ethiopia," AFP quoted the letter as saying.

BBC Amharic has contacted the Ethiopian foreign ministry to confirm the authenticity of the letter, but it has not yet responded. The letter is widely circulating on social media among Ethiopian government supporters.

A TPLF faction, led by Debretsion Gebremichael, controls the government in Ethiopia's Tigray region, which borders Eritrea.

It accuses Abiy's government of failing to fully implement a peace deal reached in 2022 to end conflict in Tigray, and says another TPLF faction, led by Getachew Reda, is colluding with Addis Ababa.

Eritrea fought on the side of the Ethiopian army in the conflict, but was not part of the deal and is now accused of forming an alliance with the Debretsion-led TPLF faction.

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At the same time, Ethiopia has been hit by an increasingly brutal conflict in its Amhara region, after the Fano militias - previously allied with Abiy's government against the TPLF - rejected moves to disarm them and said they were defending the Amhara ethnic group from government atrocities.

The Fano appear to be expanding their agenda to topple Abiy's government, and are carrying out more deadly operations.

Ethiopia's privately owned Addis Standard newspaper reported that the foreign minister's letter accused Eritrea and TPLF hardliners of supporting Fano's efforts to capture the town of Woldiya in the Amhara region in September.

TPLF commanders and fighters participated directly in the operation, the newspaper quoted the letter as saying.

The minister said Eritrea was attempting to destabilise Ethiopia as it felt threatened by Addis Ababa's bid to gain access to the Red Sea, the newspaper reported.

Since October 2023, landlocked Ethiopia has been pushing for renewed access to the Red Sea, arguing that it had erred in relinquishing control of ports to Eritrea at the time of its independence.

In Ethiopia's parliament on Monday, President Taye Atske Selassie described the Red Sea and River Nile as "great water resources, which are essential to our country's existence".

This led to a sharp rebuke from Eritrea's Information Minister Yemane Gebremeskel, who dismissed the rhetoric as "too crass and pathetic to sell".

He added that the Ethiopian ruling party's "obsession" with the Red Sea and River Nile was "bizarre and mind-boggling by all standards".