* Top officer says Pakistan army backing Kashmir incursions
* Biggest incursion in years as ceasefire frays
* Tensions building up ahead of Afghan withdrawal
* Pakistan says talks necessary to defuse tensions
NEW DELHI/SRINAGAR, India, Oct 8 (Reuters) - India's army said on Tuesday that Pakistani troops had helped a group of 30 to 40 insurgents stage the biggest incursion in Kashmir in ...