Montenegrin woman casts her ballot at a polling station in Podgorica

Monitors say voters in Montenegro were casting ballots in near record numbers Sunday in a referendum on whether to maintain their union with Serbia or become Europe's newest independent state.

Authorities say more than 50 percent of Montenegro's 485,000 eligible voters had cast ballots in the first five hours of polling.

The three-year-old Serbia and Montenegro union is the last remaining federation of former Yugoslav states.

Slovenia, Croatia, Bosnia and Macedonia broke away during the bloody Balkans wars of the 1990s.

The European Union helped organize the vote and says support from 55 percent of the electorate is needed to pass the independence referendum.

Serbian Prime Minister Vojislav Kostunica has urged Montenegrins not to break up the union, citing concerns for the minority Serb population if the federation dissolves.

But Montenegrin Prime Minister Milo Djukanovic says the current relationship between Serbia and Montenegro is not working. He says Serbia has not respected its international obligations.

Opponents argue that Montenegro, with a population of 620,000, is too small to be a viable independent state.