Even as residents who fled the city ahead of Hurricane Gustav continued to return, Mayor Ray Nagin said Saturday that it appeared the city would need to start worrying about Hurricane Ike.

Hurricane Ike grew to Category 4 strength Saturday and could head into the Gulf of Mexico by early next week, putting residents along the Gulf Coast on alert less than a week after Gustav made landfall in south Louisiana.

Nagin told reporters he's worried about fast-moving Ike and about the wherewithal of residents who one week ago began leaving ahead of Gustav. Reactions of residents who've returned have ranged from relief to find their homes not flooded to frustration with the cost of evacuating and time away from home.

"Our citizens are weary and they're tired and they have spent a lot of money evacuating, some of them, from Gustav," Nagin said Saturday evening. "My expectations this time is, it will be very difficult to move the kind of numbers out of this city that we moved during Gustav."

An estimated 18,000 residents relied on government-provided buses, trains and planes to evacuate New Orleans. On Monday, when Gustav made landfall as a Category 2 storm in south Louisiana, police estimated that only 10,000 people remained in the city out of an estimated 310,000 to 340,000 residents.

Nagin's emergency preparedness director, Jerry Sneed, said the city will have buses and trains at its disposal, should it need to evacuate people. Sneed said that if anyone is reluctant to leave -- if the call comes to evacuate again -- they should remember the images of Hurricane Katrina, which included people stranded on rooftops, surrounded by floodwaters.

"If you look at being inconvenienced by a shelter or a long bus ride, you need to remember what Katrina was like," Sneed said. "Nothing can compare to that."

At this point, the city is monitoring Ike and its still-uncertain path.

Nagin said officials are faced with a "new set of challenges," including the possibility that evacuees will not be able to rely on many of the in-state shelters at their disposal for Gustav.

The mayor said officials will need to get a good assessment on the condition of the city's levee protection system and that the potential exists for phased evacuations -- or, the evacuation of certain parts of the city considered more vulnerable to a storm.

"We may have to rely upon our levee protection system this time in a way that we didn't with Gustav," he said.