Four elite Russian policemen were killed in an explosion on Friday when their team was called to a house near the separatist region of Chechnya, a police source said. The police source said two other members of the OMON police unit summoned to the house in Ingushetia, on Chechnya\'s western border, were in serious condition. \"Information was received that representatives of illegal armed groups were in the house and an OMON group was sent there,\" the source told Reuters. \"For the moment, it is not clear just what exploded and different versions are being examined, including gas. But the main version is that of a planned attack.\" Clashes between security forces and armed guerrillas occur frequently in Ingushetia. Russian forces are subject to constant attack in Chechnya itself, despite a Kremlin plan to end a decade of separatist violence based on last month\'s election of a regional president. Russian media earlier reported that forces had freed in a \"special operation\" two prosecutors, held for about a year in Chechnya. Russia sent back troops to Chechnya in 1999, three years after it withdrew in humiliation from the region on its southern flank and allowed a separatist administration to take over. Moscow says it controls most of the mountainous area and has systematically refused to include separatists in any peace plan. (Reuters)