Syrian opposition fighters assigned to guard thousands of ISIS captives are rushing to the border ahead of an expected attack by Turkish forces, a top Kurdish general told NBC News on Monday.

Gen. Mazloum Kobani, commander of the Syrian Democratic Forces, said watching over the ISIS prisoners locked up in Syria is a "second priority" now that the United States has cleared the way for a Turkish assault likely targeting the mostly Kurdish forces along the border.

"This is a very big problem," Kobani told NBC News. "Nobody has helped in this regard."

The detention centers hold 12,000 suspected terrorists swept up during the U.S.-led campaign against the Islamic State militant group's fighters in the region, according to Kobani and U.S. officials. Of the 12,000, 2,000 are foreign fighters, and Iraqis and Syrians make up the remaining 10,000, Pentagon officials say.

Kobani said fighters who were previously tasked with securing the detention facilities are now streaming toward the border in preparation for battle with the Turkish army.

"All their families are located in the border area," he said, speaking through a translator. "So they are forced to defend their families."

Members of the Syrian Democratic Forces (SDF) gather at the al-Tanak oil field as they prepare to relaunch a military campaign against the Islamic State (IS) group, near Abu Kamal, province of Deir Ezzor, eastern Syria on May 1, 2018. Delil Souleiman / AFP - Getty Images file

Kobani spoke to NBC News just hours after the Trump administration announced that Turkey will soon launch its “long-planned operation” in northern Syrian and the U.S. won’t intervene. By withdrawing from the area, the U.S. will leave behind the SDF they have partnered with for several years, leaving them to fight Turkey alone.

“Honestly, it makes us disappointed,” Kobani said, adding that the decision hurts Syrian confidence in the U.S. and hurts American credibility.

Turkish President Recep Tayyip Erdogan has long been threatening to invade northern Syria to attack the Syrian Kurds stationed near the border.

Erdogan views the Syrian Kurds, who make up the majority of the SDF, as terrorists who threaten the stability of his country. But the Pentagon has counted the well-trained Kurdish fighters as its most effective partner in the fight against ISIS.

As a result, U.S. military officials had been urging the White House not to abandon the Syrian Kurds under pressure from Turkey.

Kobani said the decision will surely cost the lives of fighters who had battled alongside American soldiers in vanquishing ISIS.

The Kurdish commander said he's now considering what would have been unthinkable a few years ago: partnering with Syrian leader Bashar Assad to fight the Turkish forces.

"This is one of the options that we have on the table," he said.

He said he is worried about the possible Turkish military action, and called on the American people to pressure President Donald Trump to help.

"The people who fought with you against international terrorism, against ISIS, are under risk right now and they are facing a big battle alone," he said.