Serbs are mourning and Croats celebrating as the Balkan rivals both mark the anniversary of a 1995 Croatian offensive that retook territories held by rebel Serbs but also triggered an exodus of hundreds of thousands of people.

The rallies in Serbia and Croatia on Thursday come amid heightened tensions that have brought relations between the countries to their lowest point since the 1991-95 war.

Croatia is marking its military operation with two-day victory celebrations, while Serbia considers the "Storm" offensive as a war crime that resulted in the exodus of most Serbs living in Croatia, in convoys on tractors and horse-drawn carts. Hundreds were killed in the process.

The opposing views reflect simmering postwar Balkan tensions, which soared recently after nationalists returned to power in both states.