A street in the northern part of New Orleans still lies under water, three weeks after hurricane Katrina hit the city. (AFP)

New Orleans Mayor Ray Nagin said on Monday he was suspending his plan to allow some residents to return to the Hurricane Katrina-ravaged city because a new storm was gathering in the Gulf Coast and may hit the city this week.

"We are suspending all re-entry into the city of New Orleans as of this moment," Nagin said at a news conference here.

Nagin said he was concerned about Tropical Storm Rita, which was gaining strength and was moving west towards the Gulf of Mexico. "If we are off, I'd rather err on the side of conservatism to make sure we have everyone out," he said.

Nagin made the announcement after repeated warnings from top federal officials that the city was unsafe.

"Now we have conditions that have changed. We have another hurricane that is approaching us," said Nagin, who also cited the city's weakened levees for the decision.

The mayor had encouraged some residents to return to the city devastated by Katrina three weeks ago, expecting about one-third of the city's nearly 500,000 residents back over the next week.

In Washington, President George W. Bush questioned on Monday Nagin's plan to allow residents to return to New Orleans.

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

Bush expressed "deep concern" about the possibility that Rita could head into the Gulf of Mexico and bring more rain on New Orleans, which he said also had significant environmental concerns.

Also on Monday, Coast Guard Vice Admiral Thad Allen, head of the federal government's hurricane response in the Gulf Coast region, said it could be days or weeks before residents could safely return to New Orleans.