Somalia has promised to defend its territory by “any legal means” and recalled its ambassador to Ethiopia after Addis Ababa struck a deal with the breakaway region of Somaliland.

Mogadishu called the surprise pact, which would give landlocked Ethiopia long-sought access to the Gulf of Aden, a “clear violation” of its sovereignty and appealed to the international community to stand by its side.

The government said it was appealing to the UN, African Union, the Arab League and a regional east African grouping, the Intergovernmental Authority on Development, among others “to stand with the right for Somalia to defend its sovereignty and force Ethiopia to adhere to international laws”.

The deal was announced late on Monday in Addis Ababa, only days after Somalia’s central government agreed to resume dialogue with the separatist northern region after years of stalemate.

Somaliland has been seeking full statehood since claiming independence from Somalia in 1991, a move fiercely opposed by Mogadishu and not recognised internationally.

The memorandum of understanding signed by the Ethiopian prime minister, Abiy Ahmed, and the Somaliland leader, Muse Bihi Abdi, would give Ethiopia access to the port of Berbera and a military base.

It is unclear exactly what Somaliland would get in return. Bihi had said the agreement involved international recognition, but a statement published on the Ethiopian prime minister’s X account made no mention of recognition, only committing to “advance mutual interests through cooperation on the basis of reciprocity”.

Ethiopian officials have yet to confirm the full details of the agreement.

In Somaliland, Ali Hassan Mohamed, the information minister, hailed the deal as a “gamechanger”.

But it sparked fury in Somalia and, in a rare display of unity among the political elite, condemnation from several former leaders.

“Somaliland is part of Somalia under the Somali constitution, so Somalia finds this step to be a clear violation against its sovereignty and unity,” the Somali cabinet said on Tuesday.

The agreement was “null and void with no legal basis and Somalia will not accept it”, it added. “In response to this, the Somali government has recalled its ambassador in Ethiopia for consultation.”

In an address to the country, Hamza Abdi Barre, the prime minister of Somalia, called for people to stay calm.

“I want to assure you that we are committed to defending the country, we will not allow an inch of land, sea and skies to be violated,” he said. “We will defend our land with any legal means possible … We must unite and forget about our differences to defend our land, integrity and sovereignty.”

The speaker of Somalia’s upper house, Abdi Hashi, a veteran MP from Somaliland who has long had a seat in Somalia’s parliament, said: “The sea of ​​Somalia is not an animal that anyone can bring to the market.”

Omar Sharmarke, a former prime minister, described the agreement as a “provocation” by Ethiopia, and Mohamed Farmaajo, a former president, called it “a “serious concern for Somalia”.

The deal comes months after Abiy said his country, Africa’s second most populous, would assert its right to access the sea, sparking concerns among its neighbours.

Ethiopia was cut off from the coast after Eritrea seceded and declared independence in 1993 after a three-decade war.

Addis Ababa had maintained access to a port in Eritrea until the two countries went to war in 1998-2000, and since then Ethiopia funnelled most of its trade through Djibouti.

Ethiopia’s economy has been constrained by its lack of access to a the Red Sea, a narrow strip of water between Africa and the Arabian peninsula. On the southern coast of the Gulf of Aden, the port of Berbera offers an African base at the gateway to the Red Sea and further north to the Suez canal.

Somaliland, a former British protectorate with 4.5 million people, prints its own currency, issues its own passports and elects its own government.

Although Somaliland has often been seen as a beacon of stability in the chaotic Horn of Africa, its quest for statehood has gone unrecognised internationally, leaving it poor and isolated.

Its challenges were brought into sharp focus last summer, when pro-unionist communities in the autonomous region’s east captured a regional capital and declared a separate administration affiliated with the federal government of Somalia.