Mexico's Caribbean resort of Cancun is struggling to remove tons of sargassum seaweed that has been washing ashore in recent weeks.

The state government of Quintana Roo said Monday it is stepping up removal efforts, and says there have been no reports of tourists cancelling visits because of the problem.

Photos issued by the Cancun city government showed piles of brown seaweed on some normally pristine white beaches. By last week, the city said it had raked or shoveled up 500 cubic meters of sargassum.

Gov. Roberto Borge says the cause of the seaweed invasion is still unknown, though it could be due to high levels of nutrients in ocean water or changes in ocean temperatures, currents or wind patterns.