Egyptian President Abdul Fattah al-Sisi has said a deal granting Ethiopia maritime access through Somaliland "is acceptable to no-one".
Speaking in Cairo next to Somalia's President Hassan Sheikh Mohamud, Mr Sisi said Egypt would not allow Somalia's security to be jeopardised.
"We are unwavering in our support for our brothers, and if they call upon us, we will not hesitate to act," Mr Sisi added.
But on Sunday, Ethiopia's national security affairs adviser Redwan Hussein said the contentious Red Sea access deal was meant for cooperation and partnership, and is not "annexation" or "assumption of sovereignty over the territory of any state".
In a statement on X, Mr Hussein said Ethiopia and Somalia "are not just neighbours who share a border but they are fraternal nations sharing a common language, culture and people".
"Our destiny is entwined and inseparable," he added.
Somalia sees Somaliland as its territory, and says the deal is illegal.
Egypt, for its part, has had problems with Ethiopia over the construction of the Renaissance Dam on the Blue Nile, a tributary of the River Nile, which it relies on for nearly all of its fresh water.
