FRANKFURT, April 11 (Reuters) - The German airline

Lufthansa on Thursday extended the suspension of

flights to Tehran due to security concerns in the Middle East,

which is on alert for Iranian retaliation for a suspected

Israeli air strike on Iran's embassy in Syria.

Lufthansa and its subsidiary Austrian Airlines are the only

two Western carriers flying into Tehran, which is mostly served

by Turkish and Middle Eastern airlines.

The region and the United States have been on alert for an

attack by Iran since April 1, when Israeli warplanes were

suspected of bombing the Iranian embassy compound in Syria.

Lufthansa on Thursday said it had suspended flights to and

from Tehran until probably April 13, extending its suspension by

two days, to avoid its crew having to disembark to spend the

night in the Iranian capital.

"Last weekend it was decided not to operate a flight to

Tehran with a layover for the crew due to the security

situation," a spokesperson said. "On the route, the crew has to

spend the night in Tehran before the return flight to Frankfurt.

We want to avoid disembarking for safety reasons."

Austrian Airlines, which is owned by Lufthansa and flies

from Vienna to Tehran six times a week, said it was still

planning to fly on Thursday but was adjusting timings to avoid

an overnight layover.

"The Austrian Airlines flight to Tehran scheduled for today

will take place, but will depart from Vienna several hours late

in order to minimise the time between landing and departure in

Tehran," a spokesperson said.

There was no immediate word from other international

airlines that fly to Tehran. Iranian air space is also a key

overflight route for Emirates' and Qatar Airways' flights to

North America.

(Reporting by Ilona Wissenbach

Writing by Madeline Chambers and Matthias Williams

Editing by Rachel More and Ros Russell)