Austria, which has led opposition within the European Union to Romania and Bulgaria joining the Schengen open-travel area, said on Monday it was willing to relent but only on plane travel and in exchange for tighter security at the EU's external border.

At a meeting of the bloc's interior ministers a year ago at which the two eastern European countries had hoped to get the green light to join the world's largest free-travel area, Austria said illegal immigration was still too high and Romania and Bulgaria needed to do more to prevent it before joining.

Austria's ruling conservative People's Party (OVP) has for years made fighting illegal immigration a key campaign issue. A parliamentary election is due next year and the far-right Freedom Party, which has a similar line on migration to the OVP, has a clear lead in opinion polls.

"Yes, I can imagine changes as far as airports are concerned for Romania and Bulgaria," Austrian Interior Minister Gerhard Karner told ORF radio, adding that he had sent a position paper to the European Commission and the ball was now in its court.

"On the other hand, there are clear demands in exchange for that, namely massively strengthening the protection of the (EU) external border, land border checks staying in place, and asylum seekers being transferred to Bulgaria and Romania," he said, referring to people who pass through there to reach Austria.

Specifically, Karner said he wanted a trebling of the number of border police and upgrades to the technical equipment deployed, particularly at Bulgaria's border with Turkey and Romania's border with Serbia.

If his demands were met, passport checks could be scrapped between Romania and Bulgaria and countries in Schengen, but only for travel by plane - a solution he referred to as "Air Schengen". (Reporting by Francois Murphy; Editing by Alex Richardson)