Investigators have yet to find any evidence of terrorism links to Tuesday's crashes of two Russian passenger jets, a Federal Security Service (FSB) spokesman said Wednesday.

    Spokesman Nikolai Zakharov said that operational mistake is the most likely cause of the crashes which killed all 89 passengers and crew members aboard the Tu-154 and Tu-134 planes Tuesday nightin southern Russia's Rostov region and the Tula region, about 200 km south of Moscow.

    "Malfunction of aircraft systems, the use of poor quality aviation fuel, violation of fueling regulations and piloting regulations are being checked," Zakharov was cited by Interfax news agency as saying.

    Another FSB spokesman Sergei Ignatchenko also confirmed the preliminary conclusion, adding that other possible causes of the crashes are under consideration, Itar-Tass reported.

    The Volga-Aviaexpress Tu-134 and the Siberia Airlines Tu-154 left Moscow's Domodedovo airport within 40 minutes of each other, heading respectively for Russia's southern city of Volgograd and the Black Sea resort city of Sochi where President Vladimir Putin is on vacation. They disappeared from radar screens almost simultaneously at around 11:00 p.m. Moscow time (1900 GMT).

    Earlier reports said the Tu-154, with 46 people on board, sent a distress signal indicating an attack or hijacking just before itcrashed. But an unidentified law enforcement source said later that it was an SOS signal and no other signals were sent.

    Domodedovo airport said in a press release on Wednesday that the two jets had undergone all necessary technical preparations before their departure in accordance with the demands of Russian aviation and security authorities.

    Most of the debris and all flight recorders from both planes have been found at the crash sites and brought to Moscow. The investigation team -- including experts from the FSB, the InteriorMinistry and the prosecutor's office -- are examining the evidence.

    Security has been strengthened at all Russia's airports and theInterior Ministry has pledged to reinforce security measures in public places.