The mayor of New Orleans today suspended a controversial plan to allow displaced residents to return to the flood-ravaged city, citing the approach of another storm this week on a path similar to that of the devastating Hurricane Katrina.

The suspension of what Mayor C. Ray Nagin called his "reentry plan" came after President Bush questioned whether it was realistic for people to start returning to New Orleans today. Bush said the administration was worried the city might not be ready yet, and he warned that a new storm named Rita could bring renewed flooding.

Nagin said in a news conference in New Orleans that the reentry program "has been going very smoothly," but that now he was "encouraging everyone to leave" because of the threat from Rita.

He said that despite advice to the contrary from federal officials, he had launched the program because "I felt it was very important for our citizens to start to come back to this city and to feel they have a city to come back to." The program targeted half a dozen New Orleans zip codes that are home to about 182,000 inhabitants. He said he was not "encouraging any citizens to bring back children" or to put themselves at risk. "We just wanted them to come back and take a peek," then decide whether they wanted to stay, he said.

"But now we have conditions that have changed," Nagin said. "We have another hurricane that's approaching us." He added, "So we've started the evacuation process as we speak." By Wednesday, he said, he plans to "get everybody moving in earnest if this storm starts to head toward us."

After a meeting of his Homeland Security Council at the White House, Bush told reporters, "The mayor has got this dream about having a city up and running. And we share that dream. But we also want to be realistic about some of the hurdles and obstacles that we all confront in repopulating New Orleans."

The Bush administration official overseeing the federal government's response in the region has warned that basic services in New Orleans have not been restored.

Nagin announced last week that New Orleans residents could start returning to the city starting today, one zip code at a time. The plan was to reopen four city neighborhoods over the next week: Algiers, Uptown, the Garden District, and finally the historic French Quarter.

"The city needs to reemerge," Bush told reporters, but he said it was "a matter of timing."

Bush's comments came after Coast Guard Vice Adm. Thad W. Allen, head of the federal government's hurricane response team, said it could still be weeks before residents could safely return because of the lack of potable water, emergency services and other problems in the city.

Allen, speaking on NBC's "Today" show, said he was "hesitant to attach an exact date" to when residents could safely return. "What we'd rather do is establish measures of effectiveness -- when you have potable water, when the 911 system is restored. . . ."

He also stressed that a new evacuation plan needs to be put in place.

"Whatever population goes into New Orleans right now, there has to be an evacuation plan on how to get them out" in case of another storm, Allen said on the morning show.

Bush said Tropical Storm Rita, which was headed toward the Florida Keys Monday and threatening to grow into a hurricane with a potential eight-foot storm surge, is causing "deep concern" for New Orleans. He said the storm could cause "more flooding" there.

If that happened, he said, he has been warned that the city's levees could be breached again.

"We're cautious about telling people to return, especially at this moment of history," Bush said.

He added that there are still significant environmental concerns.

Bush said Nagin needed to "hear" what Allen said, as did the residents of New Orleans. The president said he would "absolutely" intervene personally with Nagin if needed -- and that he meant for his comments to make sure Nagin got the point.

Bush's and Allen's comments came as residents began trickling back into the water-damaged city.

John Schwab, 31, arrived back in Algiers with his brother and encountered no checkpoint getting into the neighborhood, according to the Associated Press. Nagin had said returning evacuees would have to produce some identification.

"I'll probably have to look for a job in construction," Schwab told the AP. "That's about the only thing around."

Many residents had already arrived. In Algiers on Sunday morning, worshipers gathered for Mass at the Holy Name of Mary Catholic Church. At the river's edge, dozens turned up for groceries distributed by troops.

Residents there said Sunday they had power, water and even cable TV. Many stoplights did not work, however, and a truck collided with an ambulance at one of Algiers' main intersections.

Dozens of residents could be seen out walking dogs, raking leaves or repairing homes on Sunday in those neighborhoods least affected by Hurricane Katrina -- Uptown, the Garden District and Algiers.

Staff Writer Peter Whoriskey contributed to this report