The Latest on the flow of migrants into Europe (all times local):

2:20 p.m.

A spokeswoman for the European Union's executive arm agrees with Hungary's electoral commission that a weekend referendum on EU migrant quotas was invalid due to insufficient voter turnout.

European Commission spokesman Margaritis Schinas said Monday that because than less than the required 50 percent of voters took part in referendum on Sunday, the vote was "declared legally void by the Hungarian electoral commission."

Hungary has staunchly opposed a Commission scheme to redistribute 160,000 refugees from overwhelmed Greece and Italy among other EU member countries.

Most of the 43 percent of voters who did cast ballots Sunday supported Budapest's opposition to future, mandatory EU moves to relocate asylum-seekers.

Schinas said: "We respect the democratic will of the Hungarian people, both of those who voted and of those who did not."

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12:55 p.m.

Serbia's president says the Balkan country will seal its borders to stop migrants if the European Union countries further along the migration route fully block their boundaries against people fleeing war and poverty.

Tomislav Nikolic said Monday that Serbia must avoid becoming a dead end from where migrants can no longer move forward. He says that at that point Serbia "will have to close its border for migrants ... because they don't want to be here."

Serbia has been toughening its policies lately after increasing numbers of migrants have entered the country looking for ways to cross into EU members Croatia or Hungary.

Serbia already has stepped up patrols along its borders with Macedonia and Bulgaria to minimize the influx. There are some 6,000 migrants currently in the country.