BUDAPEST/VIENNA (Reuters) - Border checks by Austria that have caused long queues on the Hungarian side of the frontier in recent days are unjustified, Hungary's Foreign Minister Peter Szijjarto said on Thursday.

Szijjarto said controls were needed on Hungary's Serbian and Croatian borders but not where it meets Austria. He said Hungary was doing its job to defend the external border of the passport-free Schengen zone to the south.

"I think what the Austrians are doing on the Hungarian-Austrian border is unjustified," he told a news conference.

Austrian police said an uptick in illegal crossings from Hungary had caused them to intensify checks on vehicles at the border starting on July 4. Both countries are in the passport free Schengen zone.

Hundreds of thousands of migrants crossed from Hungary into Austria last year in an unprecedented influx to Europe of people fleeing war and poverty in the Middle East and elsewhere. Most of them went on to Germany or Sweden.

After the Austrian police stepped up controls, the Hungarian police in turn introduced its own checks, causing queues to form on the Austrian side.

"We take notice of the Hungarian explanation that these are traffic controls, but in effect this amounts to border controls," said a spokesman for the interior ministry in Austria.

Austria introduced temporary border checks last year after 71 migrants were found suffocated in a truck near the border with Hungary, abandoned by people smugglers.

The spokesman said Austria's border checks were approved by the European Commission, whereas Hungary had no such agreement.

Hungary has taken an anti-immigration stance during the crisis, closing its southern border to migrants in October. This means the flow through to Austria has slowed to a steady trickle.

Austria's Interior Minister summoned Hungary's ambassador for a meeting on the issue on Wednesday. A police spokeswoman said Austria's controls would remain in place at least for the coming days.

(Reporting by Krisztina Than and Shadia Nasralla; Editing by Raissa Kasolowsky)