Biden administration officials have assessed that Russian President Vladimir Putin may use chemical weapons in Ukraine first before resorting to a nuclear confrontation with NATO if his troops continue to lose ground.The concerns, according to six people with knowledge of the matter, come amid a push by the administration to ensure allies are prepared for such an event, as well as to mobilize new resources and investments in manufacturing of detection systems for when the chemicals are used. The people, who spoke on condition of anonymity to discuss sensitive, internal deliberations, include U.S. and Defense Department officials.