BUDAPEST (Reuters) - Hungary could block Croatia's accession to the European Union's passport-free Schengen zone if it fails to defend the bloc's external borders, a senior adviser to Prime Minister Viktor Orban said.

"If Croatia puts up its hands and says, no, I don't want to defend the borders, then Hungary can only say that it isn't ready to join Schengen when the moment comes to decide," Antal Rogan, head of Orban's political cabinet, told InfoRadio, a pro-government rolling news station.

The two countries have been embroiled in an increasingly ill-tempered war of words in recent days as thousands of migrants fleeing war and poverty in the Middle East mass in the region awaiting passage to Western Europe.

Croatia said on Saturday it had "forced" Hungary to take in thousands of migrants and would continue sending them to its northern neighbor, amid deepening discord and disarray in Europe over the biggest westwards migration in decades.

Over 20,000 migrants, many of them Syrian refugees, have streamed into Croatia since Hungary on Tuesday barred their route to the European Union through its southern border with Serbia with a metal fence, tear gas and water cannon.

"A condition for entering the Schengen zone is that the country commits itself to defending Europe's external frontiers," Rogan said in the radio interview late on Friday. "If it so visibly fails to do this, then it's not ready for Schengen membership."

(Reporting By Thomas Escritt; Editing by Clelia Oziel)