BRIEF-NAFTA countries say began work to advance negotiating texts

* Mexico to host second round of NAFTA negotiations Sept 1-5 -- joint statement from U.S., Canadian, Mexican trade officials

* NAFTA countries say negotiations will continue at rapid pace, moving to Canada in late September, returning to U.S. in October, additional rounds planned for later this year

* NAFTA countries say committed to 'ambitious outcome', a great deal of effort and negotiation required in coming months

* NAFTA countries say began work to advance negotiating texts, agreed to provide additional text, comments or alternate proposals during next two weeks

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