Four Russians soldiers were killed and at least 11 injured in a helicopter crash in separatist Chechnya, Interfax reported. \"The helicopter suffered a catastrophe a few seconds after liftoff,\" the news agency quoted air force spokesman Alexander Drobyshevsky as saying. Drobyshevsky said officials were not yet sure what caused the crash outside the breakaway republic\'s second largest city Gudermes, but did not rule out that it was shot down by a rebel missile. However Denis Vyazemtsev, a spokesman for Chechnya\'s security department, told Interfax that technical failure likely caused the chopper to crash shortly after take-off. Russian military helicopters have regularly been the target of guerrilla attacks since war between federal troops and separatist rebels broke out in October 1999. News of the crash came one day after suspected Chechen rebels carried out a major suicide attack in the Russian capital, killing at least 16 other people at an open air rock concert. (AFP)