Sudan has warned Ethiopia to pull back troops it said were sent across their border or face the consequences of its “aggression”, the foreign ministry said on Sunday.

The harshly worded statement coincided with reports that Ethiopian forces backed by heavy weapons were massing on sections of the border with Sudan’s Qadaref region.

Thousands of residents fled the area to take refuge deeper inside Sudan due to fears hostilities could break out, according to Sudanese provincial officials in the area.

“Ethiopia’s aggression against Sudanese territory is an unacceptable and regrettable escalation that will have grave repercussions on security and stability in the region,” said the Sudanese statement issued late on Sunday.

“Sudan holds Ethiopia fully responsible for the consequences of its aggression...and demands that it immediately ceases aggression on its territory.”

The statement and the reported massing of Ethiopian forces on the border mark the latest flare-up in a simmering crisis between the two African nations over their porous border.

Tension is also fuelled by a dispute over the impact on Sudan of a massive dam being built by Ethiopia on the Nile less than 20 kilometres from the Sudanese border.

The border crisis has repeatedly spilt into deadly clashes over recent weeks. It began when Sudan’s military moved in December to wrestle back control of areas inside its territory that had long been settled by farmers mostly from Ethiopia’s powerful Amhara ethnic group who enjoyed the protection of federal forces and allied militias.

Sudan has vowed to regain control of similar other enclaves in the area and invited Addis Ababa to negotiate the placement of border signs according to a 1902 demarcation agreement reaffirmed in 1972.

The Sudanese military has meanwhile been building roads and erecting military-style bridges in the area to facilitate movement of its troops and armour if large-scale hostilities break out.

The border crisis has also whipped up nationalist sentiments in Sudan, with popular initiatives under way to collect money and material donations to use in improving the quality of life for border communities.

Ethiopia, for its part, said it would only negotiate if Sudan pulled its troops from the areas it retook and accused Sudan of unspecified border incursions.

It said the timing of Sudan’s move to take back the border enclaves was designed to take advantage of its military’s preoccupation with the ongoing fight against separatist rebels in the Tigray region.

There was no comment immediately available from the Ethiopian government on Sudan’s foreign ministry statement.

But Ethiopia’s foreign ministry repeated on its Facebook page on Saturday that third-party mediation was not needed to resolve the border crisis if “the Sudanese army evacuates the area it has forcefully occupied.”

South Sudan has been trying to mediate between the two nations to avert a war. A presidential envoy has extended an invitation to Gen Abdel Fatah Al Burhan, Sudan’s de facto president, and Ethiopian Prime Minister Abiy Ahmed to meet in the South Sudanese capital Juba to try to find a resolution. It is not known whether either leader has agreed to go and no date has been set for the meeting.

Ethiopian refugees who fled fighting in the Tigray Region flee to Sudan - in pictures