NEW ORLEANS — Army engineers trying to plug New Orleans' breached levees struggled to move giant sandbags and concrete barriers into place, and the governor said Wednesday the situation was growing more desperate and there was no choice but to abandon the flooded city.

"The challenge is an engineering nightmare," Gov. Kathleen Blanco said on ABC's "Good Morning America."

As the waters continued to rise in New Orleans, the Pentagon began mounting one of the biggest search-and-rescue operations in U.S. history, sending four Navy ships to the Gulf Coast with drinking water and other emergency supplies, along with the hospital ship USNS Comfort, search helicopters and eight swift-water rescue teams. Red Cross workers from across the country converged on the devastated region.

The Army Corps of Engineers said it planned to use heavy-duty Chinook helicopters to drop 3,000-pound sandbags Wednesday into the 500-foot gap in the failed floodwall. But the agency said it was having trouble getting the sandbags and dozens of 15-foot highway barriers to the site because the city's waterways were blocked by loose barges, boats and large debris.

Officials said they were also looking at a more audacious plan: finding a barge to plug the 500-foot hole.

The death toll from Hurricane Katrina reached at least 110 in Mississippi alone, while Louisiana put aside the counting of the dead to concentrate on rescuing the living, many of whom were still trapped on rooftops and in attics.

The Red Cross reported it had about 40,000 people in 200 shelters across the area in one of the biggest urban disasters the nation has ever seen.

A full day after the Big Easy thought it had escaped Katrina's full fury, two levees broke and spilled water into the streets Tuesday, swamping an estimated 80 percent of the bowl-shaped, below-sea-level city, inundating miles and miles of homes and rendering much of New Orleans uninhabitable for weeks or months.

"We are looking at 12 to 16 weeks before people can come in," Mayor Ray Nagin said on ABC's "Good Morning America, "and the other issue that's concerning me is we have dead bodies in the water. At some point in time the dead bodies are going to start to create a serious disease issue."

Blanco said she wanted the Superdome — which had become a shelter of last resort for about 20,000 people — evacuated within two days, along with other gathering points for storm refugees. The situation inside the dank and sweltering Superdome was becoming desperate: The water was rising, the air conditioning was out, toilets were broken, and tempers were rising.