The transaction is subject to regulatory approval, BC Partners, an international investment firm, said in a statement.
"KKR will retain a substantial minority stake," the statement reads
The price of the transaction was not disclosed.
According to The Wall Street Journal, which cited "people familiar with the matter", the deal values United Group at 2.6 billion euro ($3 billion).
United Group's fibre and cable networks cover 1.82 million homes across SEE, according to the statement.
United Group offers telecommunications and media services in Bosnia and Herzegovina, Croatia, Macedonia, Montenegro, Serbia and Slovenia, according to information published on the website of the company.
($ = 0.8570 euro)