TOKYO, March 17 (Reuters) - Japan's Nikkei share average stumbled to a 6-week low on Monday as escalating tensions in Ukraine soured investor sentiment, although SoftBank Corp jumped after its affiliate said it would go public in the United States. The Nikkei slipped 0.4 percent to 14,277.67, hitting its lowest closing level since February 6. The benchmark tumbled 3.3 percent on Friday and shed 6.2 percent last week, marking the biggest weekly drop since last June. Index heavyweight SoftBank climbed 4.9 percent after its Chinese e-commerce affiliate Alibaba Group Holdings decided to hold its long-awaited share listing in the United States. It was the most traded stock by turnover on the main board. The broader Topix index dipped 0.8 percent to 1,154.93 in subdued trade, with 1.99 billion shares changing hands. The JPX-Nikkei Index 400, a gauge comprising firms with high return on equity and strong corporate governance, lost 0.9 percent to 10,441.07.