TOKYO, March 3 (Reuters) - Japan's Nikkei average slid to a 1-1/2-week low on Monday as escalating tensions in Ukraine sparked risk aversion and a stronger yen hurt the mood, but the oil sector bucked the trend after crude prices rallied to multi-month highs. The Nikkei ended 1.3 percent lower at 14,652.23, its lowest close since Feb. 20. The index extended losses into a fourth day. Ukraine mobilised for war on Sunday and Washington threatened to isolate Russia economically after President Vladimir Putin declared he had the right to invade his neighbour, in the biggest confrontation between Russia and the West since the Cold War. Russia is the world's biggest oil producer and Ukrainian Prime Minister Arseny Yatseniuk said Moscow's move to use military force was a "declaration of war". The mining subsector gained 1.0 percent. The Topix dropped 1.2 percent to 1,196.76. The JPX-Nikkei Index 400, an index launched this year comprising firms with high return on equity and strong corporate governance, dropped 1.3 percent to 10,831.08.