Officials said the fire had “grown considerably” on Monday and was now burning on 25,000 acres.
Texas Governor Rick Perry has cut short presidential campaigning, and said the next 48 to 72 hours would be “crucial”.
Recent fires across Texas have burned 3.5 million acres, Mr Perry said, adding: “That’s roughly the size of Connecticut.”
He urged people to heed evacuation orders and not to stay in their homes if they had lost power.
“I understand that losing your home or lifetime possessions is incredibly difficult, but do not put your life in jeopardy,” Mr Perry said.
Around 25 fires are burning across Texas, which has been hit by high winds generated by Tropical Storm Lee.
A blaze in east Texas killed a 20-year-old woman and her baby daughter on Sunday before it was extinguished.
An estimated 5,000 people have been forced to leave their homes because of the Bastrop fire, which is moving away from the state capital of Austin.
Earlier, Jan Amen of the Texas Forest Service described the central Texas blaze as “a monster” that had been “zero percent contained”.
The fire is the largest of dozens of wildfires burning throughout the state, including more than 60 that started on Sunday.
Texas has been suffering its worst drought since the 1950s.