The signal came at 11:04 p.m. (1904 GMT) Tuesday from the Tu-154 airliner that disappeared in southern Russia's Rostov region, the report quoted a Russian official as saying.

    The plane, with 44 people on board, went missing from radar screens en route Moscow to the Black Sea resort city of Sochi.

    Earlier, the Itar-Tass news agency reported that the Tu-154 crashed near the city of Rostov-on-Don south of Moscow.

    It lost contacts at 22:59 Moscow time (1859 GMT) when it was expected to be 140 km from the city, said the report.

    Almost the same time, another passenger jet Tu-134, with 44 people on board including eight crew members, crashed in the Tula region south of Moscow, killing all the people on board, said the Russian emergency ministry.

    The rescuers have found the plane's tail and were searching for other pieces, flight recorders, as well as possible survivors.

    Report said the plane took off Moscow's Domodedovo airport and headed for Volgograd at 22:32 Moscow time (1832 GMT). Communication with the aircraft was lost at 22:59 Moscow time (1859 GMT).

    Witnesses said they saw an explosion on board the plane just before it crashed.

    Russian President Vladimir Putin has ordered the Federal Service of Security to investigate the two incidents immediately, Putin's press secretary Alexei Gromov said.

    Meanwhile, security has been tightened at all the country's airports after the crashes, as the Russian authorities have not ruled out terrorist attacks against the aircraft.

    A presidential election was scheduled to be held in Chechnya, a breakaway republic of Russia, on Sunday, and separatist rebels there have been blamed for a series of terror strikes that have claimed hundreds of lives.