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Moscow on Sunday vowed to respond to a "dangerous escalation" of violence near its border after authorities said a Russian man was killed by a shell fired from inside Ukraine. Russia said it has launched a probe to investigate the "murder" after a shell hit a residential area of the town of Donetsk, which lies next to the border with Ukraine's restive east. Deputy foreign minister Grigory Karasin said the incident represented a "dangerous escalation for our citizens" and vowed it "will not be left without a response," in televised remarks. The small Russian town of Donetsk is near the border with Ukraine's Lugansk region, where Kiev-backed troops and pro-Russian separatists are fighting for the control of border checkpoints.