Wildfires rolled across dry terrain in the Western United States and through a region of Canada, fueled by lightning, whipping winds and temperatures in the triple digits that have fanned the flames across parched lands.

The National Weather Service early on Monday issued red flag warnings, signaling critical conditions over the next 24 hours, for regions in at least six states. In some areas, firefighters have been battling fires for a week, trying to keep them from spreading across roads and churning through dry vegetation near populated areas.