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)