BUCHAREST (Reuters) - Romania had a legitimate expectation to join Europe's open-borders Schengen area, Prime Minister Nicolae Ciuca said on Wednesday, rejecting Austrian claims that it is a gateway for illegal migrants as unjustified.

Austria has said it opposes expanding Schengen to Romania and also Bulgaria, saying the number of illegal migrant arrivals passing through would have to fall for it to agree.

Data from EU border agency Frontex showed illegal migrants were predominantly entering the EU from the Western Balkans, not Romania.

"Practically, we cannot speak of a real justification at this moment," Ciuca said.

"We all understand that the illegal immigration problem is politically sensitive in many member states, but blocking Romania from joining Schengen will not bring Austria the answers it wants."

The Netherlands, which had previously opposed Romania's Schengen bid now endorses it, although it remains opposed to Bulgaria's entry.

A previous vote held in 2011 was blocked by several states citing concerns about corruption and organised crime.

Romania and Bulgaria are on major illegal trade routes to Europe for arms, drugs and human trafficking, but the European Commission said they had been thoroughly investigated and met all Schengen requirements.

"We have objective arguments about ... Romania's readiness to join Schengen, and our commitment to contribute to European security is firm," Ciuca said. "We have proved it over the years and more recently in the context of the crisis in Ukraine."

EU and NATO member Romania shares a 650-kilometre (400 mile) border with Ukraine. Just under three million Ukrainians have fled to Europe through Romania in the nine months since Russia invaded the country.

Ukraine has also sent roughly 8 million tonnes of grain to the Romanian Black Sea port of Constanta since Feb. 24.

(Reporting by Luiza Ilie; editing by Kirsten Donovan)