(Adds confirmed Dallas cases)
By Jim Forsyth
SAN ANTONIO, April 27 (Reuters) - Texas state health authorities on Monday confirmed four more cases of swine flu that is threatening to become a pandemic, bringing the state's total to six.
To contain the outbreak, Texas officials closed 14 schools near San Antonio and one in Dallas.
In North Texas, Dallas health officials are tracking three more "probable" swine flu infections that could be confirmed by the Centers for Disease Control and Prevention in coming days.
In Guadalupe County near San Antonio, all three confirmed cases involve students at Byron Steele High School, the Texas Department of State Health Services said.
The cases in Dallas affect a wider age group -- a 24-year-old, a 7-year-old and a 3-month-old, according to Dallas County Health and Human Services officials.
The new virus has killed up to 149 people in Mexico and there were 40 confirmed U.S. cases, though there are no confirmed U.S. deaths from the virus that spreads quickly between humans. [nN27539441]
The World Health Organization on Monday raised its pandemic alert level to phase 4, indicating a significant increased risk of a pandemic, a global outbreak of a serious disease. It would be the world's first flu pandemic since 1968.
Elsewhere in Texas, local officials said they were tracking four possible cases in Nueces County near Corpus Christi.
In Schertz, Texas, two high school students have recovered and the third is recovering under voluntary isolation at his home, officials said.
In the middle class suburb outside San Antonio where the three confirmed swine flu cases live, residents did not show any alarm.
"It doesn't make me nervous, it's only three cases so far," said Yvette Sosa, whose daughter attends Byron Steele, one of 14 area schools that authorities closed indefinitely as a precaution. "I'm not scared yet."
(Editing by Vicki Allen)