NEW YORK — New York City, still reeling from the impact of Hurricane Sandy, will expand its evacuation zones, tighten building codes, and look for ways to fortify critical infrastructure like transportation and electrical networks from future natural disasters, Mayor Michael R. Bloomberg said Thursday.

But while the mayor said he would aggressively pursue a rebuilding of the city’s damaged waterfront, he warned that ‘‘there are no panaceas or magic bullets’’ to fully protect the city. And while he did, for the first time, specifically suggest dunes or levees as protective measures worth exploring, he again dismissed the use of expensive experiments like sea gates stretching across New York Harbor.