Water is pouring over a patched-up levee in New Orleans, increasing fears that rains brought by Hurricane Rita could flood the city again.
Army engineers have warned that flood defences damaged by Hurricane Katrina can only cope with 15cm (6in) of rain.
A stream of water at least nine metres (30ft) wide was reported to be spilling into the low-lying Lower Ninth Ward.
About two million people have fled the Texas and Louisiana coasts ahead of Hurricane Rita's arrival.
The storm has weakened to a Category Three hurricane, but still boasts sustained winds of 125mph (201km/h).
It is expected to make landfall in Texas late on Friday or early on Saturday local time.
It was centred about 220 miles (354km) south-east of Galveston, Texas, at 1500 GMT, the US National Hurricane Center said, and was roaring towards land at about 10mph (16km/h).
The Gulf Coast as far west as Mexico is on alert, with officials warning Rita's course remains unpredictable. Forecasters say the hurricane could weaken to a Category Three when it reaches land.
In other developments:
- President George W Bush is to fly to Colorado to monitor hurricane preparations, instead of an earlier planned visit to San Antonio, Texas
- National Guard lorries are taking badly-needed fuel to petrol stations and stranded motorists in Texas
- More than 99% of oil output is shut off in the Gulf of Mexico, and gas production is also severely affected.
The exodus from the Gulf Coast has been slowed by huge traffic jams, with some motorists forced to abandon their cars after running out of fuel.
Up to 24 elderly evacuees died when the bus they were travelling in caught fire on a gridlocked motorway carrying traffic from Houston to Dallas.
Television pictures showed the charred shell of the bus, which was hit by a series of explosions apparently caused by oxygen containers for those on board.
THE SAFFIR-SIMPSON SCALE
US National Hurricane Center
Officials said congestion within Houston had cleared but vehicles were still stuck bumper-to-bumper on highways further inland.
People who have not left the city - among them motorists who turned back after sitting in traffic jams for hours - are now being advised to stay at home.
"Those people at risk should not get on the highways to evacuate," Houston Mayor Bill White said. "People should prepare to shelter in place if they have not evacuated."
The few residents who had returned to New Orleans, battered by Hurricane Katrina last month, were ordered to leave ahead of the latest storm.
High winds propelled by Hurricane Rita are causing a storm surge in the city, with water levels whipped up above the height of the damaged defences.
Water levels in New Orleans are rising faster than expected
At least a foot of water is now pouring through the Lower Ninth Ward area, which had only just been pumped dry following the flooding after Katrina.
A spokesman for the US Army Corp of Engineers (USACE) confirmed that flood waters were over-topping a levee on the Industrial Canal.
Col Richard Wagenaar told the BBC the water level in that area was rising much faster and higher than expected.
"The water's come up to a much higher elevation than we thought it would this early in the storm," he said.
"We were expecting a storm surge of two to five feet tomorrow morning - and right now in places we've got six feet."
Col Wagenaar said the priority for the engineers was to strengthen the repairs on Industrial Canal and maintain the patched-up breaches on the 17th Street and London Avenue canals.
The BBC's Claire Marshall in New Orleans says some people are reporting floodwater up to waist level in parts of the city, only days after engineers pumped it dry.