After group captures 7 of 34 provincial capitals, US envoy Zalmay Khalilzad tells insurgents a military takeover of Kabul would guarantee international isolation

US to draw forces down to 8,600 from 13,000 with remaining troops withdrawing in 14 months, threatens to 'nullify deal if commitments not held

No immediately claim of responsibility for attack, in which 15 are wounded, most in critical condition