Police in Austria rejected Romania’s claim that it was blameless in a European migration crisis.
It came after Austria announced new joint patrols with Hungary to police the EU’s frontier with the Balkans.
Landlocked Austria claims 75,000 unregistered migrants have arrived at its borders this year.
It used that as justification to veto Romania and Bulgaria’s membership of the visa-free Schengen zone in a vote last week.
Romania angrily hit back, saying it had nothing to do with the Balkan route and accusing Austria of playing political games.
A 20-point EU plan on Balkan migration that was published last week also made no mention of Romania.
But Austrian authorities claim 15,000 people have taken a route through Serbia, Romania and Hungary.
Brigadier Gerald Tatzgern, from Austria’s criminal police, said figures could be disputed because asylum seekers were not being registered in Romania.
“We have intercepted more than 75,000 people in Austria who are not registered anywhere in the EU,” he told OE1 radio on Monday.
“If nobody knows that these people were trafficked through their territory, they cannot be included in statistics.”
Austria says the unregistered migrants are a security risk and that Schengen should not be expanded until the problems are fixed.
Chancellor Karl Nehammer announced on Sunday that Austrian police would join Hungarian patrols on the Serbian border.
Romania and Bulgaria have also been offered Austrian support despite the diplomatic spat.
“If I want a Schengen zone, I need to have strong external borders so that the internal borders can be dismantled,” Mr Nehammer said.
“That hasn’t been working for a while. Austria has a greater burden from the migration problem than other EU countries.”
The government says many people are filing hopeless asylum claims, coming from countries such as India and Tunisia that Austria regards as safe.
Austria’s figures showed 15,020 Indians and 11,394 Tunisians claimed asylum from January to October, compared to only a few hundred last year.
However, the top countries represented were Afghanistan with 20,633 asylum seekers and Syria with 15,953.
There were thousands each from Pakistan, Morocco and Turkey, and fewer from Somalia, Egypt and Bangladesh.
The figures showed an increase of more than 200 per cent in total asylum claims since last year.
Austria has pressured Serbia to block visa-free travel from Tunisia and India to close off one route to Europe.
It claimed vindication after European Commission President Ursula von der Leyen said Austria was exceptionally hard hit by the crisis.
But there was much dismay at its veto of Romania and Bulgaria, with Austria accused of jeopardising EU unity.
Romania summoned Austria’s ambassador on Thursday to object to the vote, in which the Netherlands also opposed Bulgaria’s entry.
There was no objection to Croatia, meaning it will join the Schengen zone on January 1.