DALLAS (Reuters) - Texas Governor Greg Abbott on Tuesday extended the deployment of National Guard troops along the state’s border with Mexico to help block illegal crossings by unaccompanied children into the United States.

Texas governor Greg Abbott reacts during a meeting with Cuban Minister of Foreign Trade and Foreign Investment, Rodrigo Malmierca, (not pictured), in Havana, December 2, 2015. REUTERS/Stringer

The deployments began 18 months ago when thousands of migrant families and unaccompanied youth began streaming over the border mainly from El Salvador, Honduras and Guatemala. They were due to be completed at the end of December.

Abbott, a Republican, also ordered an increase in boat patrols on the Rio Grande River and air surveillance. Earlier this year, the state legislature approved an $800 million two-year plan to add 250 public safety officers to the border region.

Despite a slowdown late last year, the U.S. Border Patrol reported that more than 10,000 unaccompanied children were apprehended crossing the U.S. Southwest Border in October and November, about double the same period a year ago.

Nearly 6,500 of the children were caught crossing into the Rio Grande Valley in Texas, compared to 3,200 a year earlier.

The U.S. Department of Health and Human Services recently opened two new shelters for the migrant children in Texas and one in California to help address the increase.

About 500 children from ages 12 to 17 are being held at a religious retreat camp in Ellis County 40 miles south of Dallas and 200 more are expected to arrive by the end of the week, the U.S. Department of Health and Human Services said.

The majority of the children are from Honduras, Guatemala and Ecuador, department spokeswoman Andrea Helling said. They can spend up to 30 days waiting for a relative to sponsor their release and receive a date for an immigration hearing, she said.

“Most of them are fleeing economic hardship and gang violence,” Helling said. “A lot of them come across the border with phone numbers of family, ready to call.”

Texas officials would not say how many National Guard troops would stay or for how long. About 1,000 troops patrolled the state’s southern border at the height of the influx in 2014, Texas National Guard Lieutenant Colonel Travis Walters said.