Somalia on Tuesday vowed to defend its territory after a controversial deal between Ethiopia and the breakaway state of Somaliland that it branded an "aggression".
The government in Mogadishu also said it had recalled its ambassador to Ethiopia and issued an appeal to the international community to stand by its side over the "blatant assault" on its sovereignty.
The deal was announced in Addis Ababa on Monday only days after Mogadishu agreed to resume dialogue with the separatist northern region after years of stalemate.
Somaliland, a former British protectorate of about 4.5 million people, has been seeking full statehood since claiming independence from Somalia in 1991.
But the move has not been recognised internationally and is fiercely opposed by Mogadishu although in reality the central government exercises little authority over the region's affairs.
The "historic" memorandum of understanding (MOU) signed by Ethiopian Prime Minister Abiy Ahmed and Somaliland leader Muse Bihi Abdi gives Ethiopia access to the Red Sea port of Berbera and a leased military base.
Abdi said in a statement that in return, Ethiopia would formally recognise Somaliland, something that Addis Ababa has not confirmed.
The Somalia cabinet denounced the MoU as a "blatant assault on the independence, sovereignty and unity of the Federal Republic of Somalia", and announced it was recalling its ambassador to Ethiopia for consultations.
The cabinet also said it was seeking urgent meetings of the UN Security Council and the African Union to discuss "Ethiopia's aggression and interference against the sovereignty of our country".
The two Horn of Africa neighbours have a history of stormy relations and territorial feuds, fighting two wars in the late 20th century.
In an address to the nation, Prime Minister Hamza Barre called for the people of Somalia to stay calm.
"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."
President Hassan Sheikh Mohamud made a similar declaration to an emergency session of parliament, rejecting the deal as a violation of international laws but insisting Somalia bore the people of Somaliland no animosity.
He warned however that the deal could open the way to a "revival" of Al-Shabaab that had been waging a bloody insurgency against the Somali government for more than 15 years.
Al-Shabaab itself, which has been the target of a major military offensive since late 2022, denounced Abiy's "expansionist agenda".
There was no immediate comment from the Ethiopian government to Somalia's reaction.
The deal comes months after Abiy said his country, Africa's second most populous with about 120 million people, would assert its right to access the Red Sea, sparking concerns among its neighbours.
Ethiopia was cut off from the coast after Eritrea seceded and declared independence in 1993 following 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 funnels most of its trade through Djibouti.
Ethiopia's economy has been constrained by its lack of maritime access, and Berbera port offers a gateway to the Red Sea and further north to the Suez Canal.
Abiy's national security adviser Redwan Hussein said Ethiopia would have a military base on the Red Sea, with Somaliland saying it would lease 20 kilometres (12 miles) of sea access for 50 years.
Somaliland would have the opportunity to take stakes in Ethiopian telecoms and airline companies, Hussein added.
It was not clear when the pact would take effect.
In 2018, Ethiopia acquired a 19-percent shareholding in Berbera port, which is managed and majority owned by Dubai-based DP World, although the deal later lapsed.
On Friday, Somalia and Somaliland agreed to resume dialogue after two days of talks mediated by Djibouti President Ismail Omar Guelleh, the first of their kind since 2020 when similar negotiations stalled.
In a on X, Guelleh's adviser Alexis Mohamed said the region "does not need other nests of conflict," without specifically referring to any country.
Somaliland prints its own currency, issues its own passports and elects its own government but has been left poor and isolated without international recognition of its quest for statehood.
Although it has often been seen as a beacon of stability in the chaotic Horn of Africa region, political tensions surfaced there last year, spilling over into deadly violence.