U.S. and Mexican forecasters said Hurricane Ike was gaining size and strength over the Gulf of Mexico on Wednesday after the storm battered parts of Cuba this week.
The U.S. National Hurricane Center in Miami upgraded Ike to a Category 2 storm Wednesday afternoon and said it would become a "major hurricane" — a Category 3 storm — by Thursday.
Mexico's National Meteorological Service said "favourable conditions exist for Ike to reach Category 3" before landfall.
The storm was expected to slam into Texas or northern Mexico on Saturday. Its most likely target is Corpus Christi, Texas.
Forecasters said in an update at 5 p.m. ET that the eye of the storm was about 375 kilometres north of Cancun, Mexico, and 590 km southeast of the Mississippi River Delta. Maximum sustained winds were said to be about 160 kilometres an hour, with gusts to 195 km/h.
Evacuation could affect 1 million
In Mexico, cities in the coastal state of Tamaulipas were put on alert and began precautionary maintenance work on sewers and canals to ensure rain and floodwaters can quickly drain.
Rescue agencies began setting up emergency shelters for people who may have to flee from the storm.
Officials in Texas began to roll out evacuation plans on Wednesday.
A total of 88 coastal counties have already been declared disaster areas by Texas Gov. Rick Perry to ensure they receive state aid if needed.
Perry has also put 7,500 National Guard troops on standby and drawn up a possible evacuation route.
Officials in Matagorda County, about halfway between Houston and Corpus Christi, began a mandatory evacuation for residents in its coastal areas.
Elsewhere, residents who have special needs were urged to move inland.
In all, officials predicted that one million people in coastal communities in Texas near the Mexican border may be forced to leave their homes.
Texas officials told residents who live in areas likely to be hit by the hurricane to keep an eye on weather reports and fill their vehicles with gas.
"We have a fuel team that is part of the state operation system in Austin," said Allison Castle, spokeswoman for Perry.
"They are helping to push fuel to hurricane areas. One of the lessons we learned from past hurricanes is we need to have fuel ready," she said.
On Tuesday, the hurricane churned across Cuba, destroying and damaging homes, killing at least four people and forcing 2.6 million people to seek refuge, according to Cuban state television. It has been blamed for at least 80 deaths in the Caribbean, mostly in Haiti.
Power had not been completely restored in Havana, the Cuban capital, as of Wednesday morning.