Brotherhood of cities as a form of cooperation began after the Second World War, when peoples in Europe realized the importance of reconciliation which could start from the local level.
In Europe, this trend is increasing and fraternal cooperation exists in 17,000 cases.

Serbian cities are most frequently twinned with cities in Greece and Hungary.

The reason of twinning can be very different. Novi Pazar is twinned with the municipalities in Turkey where many citizens of Novi Pazar live, the brotherhood of some cities was due to the common victims in the Second World War, because of mutual assistance in wartime or some other similarities in the sufferings. Some twinning and friendly relations between cities have unfortunately extinguished during the last wars.

Interestingly, for example, Belgrade signed the first charter of friendship in 1957, with Coventry from the United Kingdom. At that time, Belgrade and Coventry, on the basis of the similarity of the victims of the bombings during the Second World War, became twin cities. The crown of this cooperation is the square in Coventry bearing its name - Belgrade. Belgrade, as a reconstruction aid, has donated to Coventry timber for the construction of the first post-war theater. As a sign of gratitude, the theater received the name Belgrade Theater, and the square on which it is located also bears the name Belgrade.

Here is the list of Serbian towns that are twinned with Greek.

Belgrade has been twinned with Corfu since 2010.
Novi Sad and Iliopoli (Athens municipality).
Šabac is the fraternal city of Argostoli (the capital of Kefalonia), in which one street has recently been named “Šabac”.

Jagodina has signed a twinning charter with three Greek cities - Corinth, Perdika, Hrisoupoli. With the charter with Hrisupoli, the municipality of Jagodina received two hectares of land by the sea as a gift.
With the charter with Hrisupoli, the municipality of Jagodina received two hectares of land by the sea as a gift.
Vranje is twinned with Greek city of Trikala, and Krusevac is twinned with city of Corfu.

Patra is twinned with Banjaluka and Aleksinac.

An interesting combination is the Greek island of Simi and Guča.
Paralia is twinned with Jagodina, Čačak with Katerini, and Čajetina with Aidipsos.

Municipality of Čukarica has signed a fraternal charter with the island of Thassos, Niš with Serres, Prnjavor with Thessaloniki, Aranđelovac and Filipi, Užice and Veroia, Niš and the municipalities of Glyfada, Čajetina and Lefkimi, Smederevo and Volos …

There are certainly cities that we missed out, so you can add them to your comment.