Almost a third of asylum seekers arriving in Germany who claim to be Syrian are not, according to officials.
Hundreds of thousands of people have come to Germany since the start of the year, seeking refuge from poverty, persecution and war.
Germany has said it will temporarily refrain from sending Syrians back to other European Union countries they have travelled through, as would normally be possible under EU rules.
This has been interpreted by some as Germany giving special preference to people from Syria, who make up the largest single group of asylum seekers.
Tobias Plate, a spokesman for Germany's interior ministry, said that it is estimated that "30% of asylum seekers claiming to be Syrian in the end aren't Syrian".
Meanwhile, European Union president Donald Tusk said Europe might need a border guard to protect its outside borders from an "excessive wave" of migrants which could be a threat to its internal open borders system.
Mr Tusk spoke on Polish state TV, hours after an EU summit decided to toughen border controls and offer more money to refugees in the Middle East as ways of coping with the migration crisis in Europe.
Calling for a "very serious, quick talk" about whether a European border guard might be needed, Mr Tusk said that without border control "we have no migration policy but rather total chaos".
The office of Austrian chancellor Werner Faymann said he and Hungarian prime minister Viktor Orban have agreed day-to-day co-operation is necessary to manage the flow of migrants through the two countries.
The office of the centre-left chancellor emphasised that Europe has common standards for accommodating, taking care of and processing asylum seekers that need to be respected.
Mr Orban has said he will hold consultations with other countries in the region and with the United Nations before making a decision about closing the border with Croatia for migrants.
He said he will seek support for a border fence and once the other countries are prepared, Hungary will enforce rules which guarantee the ways of crossing the Croatian-Hungarian border "are in line with European Union law".
Mr Orban had earlier said he expects the fence being built on the border with Croatia to be completed within days.
Later, Croatia said it has lifted its blockade of the border with Serbia after a spat between the long-time Balkan rivals over a migrant surge.
Croatian police said the border is now open "to all traffic without restrictions".
The two nations had traded accusations over how to manage the tens of thousands of migrants crossing their territories to get to Western Europe, imposing border restrictions on cargo and civilian crossings.
EU officials had urged Croatia to lift the blockade that has cost the two nations millions of dollars.
Serbian interior minister Nebojsa Stefanovic welcomed Croatia's decision to reopen the border.
He called it "a great victory for all the citizens of Serbia".