LONDON, March 2 (Reuters) - President Vladimir Putin said on Thursday Russia had been hit by a "terrorist attack" in the southern Bryansk region bordering Ukraine, and vowed to crush what he said was a Ukrainian sabotage group that had fired at civilians. In two videos circulating online, armed men calling themselves the "Russian Volunteer Corps" said they had crossed the border to fight what they referred to as "the bloody Putinite and Kremlin regime. In December, the FSB said a "sabotage group" had been "liquidated" while trying to enter the region