MEXICO CITY (CBSDFW.COM/AP) — A caravan of about 1,700 Central American migrants has arrived at the Mexican border city of Piedras Negras, just west of Eagle Pass, Texas.

While previous caravans had preferred the border city of Tijuana, the relatively open section of the border around Eagle Pass is marked mainly by the Rio Grande River and lacks the long sections of high barriers found in Tijuana.

Still, the U.S. Department of Homeland Security vowed Tuesday that the “lawless caravan” would not be allowed in.

“Approximately 2,000 [illegal] aliens have arrived in northern Mexico as part of a ‘caravan’ seeking to cross the border into Texas. Illegal entry will not be tolerated and we stand ready to prevent it,” DHS Secretary Kirstjen M. Nielsen wrote in a statement, adding “DHS will take all steps to ensure the safety and security of law enforcement personnel on the frontlines.”

Border Patrol agents are on high alert and a number of bridges are closed. Images from local media show U.S. agents with riot gear and shields standing on a bridge separating Eagle Pass and Piedras Negras. DHS said Border Patrol agents have already apprehended some migrants who crossed the border illegally Monday night.

Coahuila state Gov. Miguel Angel Riquelme said about 1,700 migrants arrived late Sunday aboard 49 buses from the cities of Saltillo and Arteaga. Another smaller group headed toward the neighboring state of Nuevo Leon.

An improvised shelter was set up for the migrants at an unused maquiladora factory, and local officials said the migrants had been given sleeping mats, blankets, food and wireless access.

State child welfare officials reported there 46 unaccompanied youths aged between 15 and 17 in the caravan.

The area from Piedras Negras east to Nuevo Laredo had long been dominated by the now-fragmented Zetas cartel.

The caravan was escorted by soldiers and police, and the state government said migrants wouldn’t be allowed to split off from the main group because “these types of caravans have been victims of organized crime groups that try to force the migrants to work for them.”

“That is why we are being strict about security,” said Coahuila state Interior Secretary Jose María Fraustro.

Local media reported that some migrants objected to guards and closed gates at the improvised shelter, saying they don’t want to be locked in.

Previous caravans of mainly Honduran migrants had headed for the border city of Tijuana last year in their bid to reach the United States.

But Tijuana officials said their city was overwhelmed and unprepared to receive more migrants.

A second, larger caravan of several thousand migrants is expected to set out across Mexico soon.

(© Copyright 2019 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.)