Burundi has not fulfilled the requirements to join the Southern African Development Community (Sadc).
The country applied to join the regional body in 2017, but its request was put on hold pending an investigation.
A Sadc team was sent to Burundi in May - and based on its findings at the head of states summit this weekend it was decided that the country needed to satisfy some further issues before it could be admitted.
Tanzanian President John Magufuli, who hosted the summit, did not specify what these requirements were.
Since 2003, Sadc has specified that countries wishing to join the body should respect principles of democracy, human rights, good governance and the rule of law.
In June, a UN human rights commission report accused the Burundian government of repressing the opposition and of "serious human rights violations".
Burundi's representatives at the UN called the report "a document of lies".