US urges Turkey to halt announced offensive into Syria that threatens US allies and ISIS jails

The U.S. is urging Turkey not to undertake an offensive into northeastern Syria against U.S.-backed Kurdish forces that helped to defeat the Islamic State and that are now maintaining prisons with thousands of the terror group's fighters.

An American military delegation is in Turkey to meet with Turkish officials and continue negotiations on an alternative to a Turkish military operation that could also threaten U.S. troops stationed in the area.

Turkish President Recep Tayyip Erdogan, who has a close relationship with President Donald Trump, announced Sunday that Turkey's military would begin the offensive imminently against the Syrian Democratic Forces (SDF), the mostly Kurdish group that served as the U.S. ground troops in the fight to destroy ISIS's caliphate in Syria. The group has ties to their Kurdish counterparts in Turkey, the Kurdistan Workers' Party or PKK, which the U.S. and Turkey consider a terrorist organization.

Turkish military forces have been increasing deployments along the Turkish side of the border in recent weeks -- including heavy weaponry, tanks and artillery -- in readiness for the attack.

While Erdogan has repeatedly threatened to undertake an offensive, this was the first time he said the plan had already been put in motion -- saying the U.S. and Russia had both been notified.

The State Department would not say whether that was true, but days earlier, a senior U.S. diplomat said there wouldn't be a Turkish offensive.

"There are no talks with the Turks on protecting the Kurds ... or stopping an invasion because we don't see an invasion," Ambassador James Jeffrey, the U.S. special envoy for Syria and to the Global Coalition to Defeat ISIS, told reporters Thursday.

The two countries have been meeting for months now since the fall of ISIS's last towns and cities to discuss the presence of U.S.-armed and backed Kurdish forces, which Turkey sees as a direct threat. While at times reaching agreements on joint military patrols or safe zones, Erdogan's announcement seemed to signal a breakdown in talks -- or public posturing to gain leverage.

In a statement to ABC News on Sunday, State Department spokesperson Morgan Ortagus called any offensive "unacceptable" and urged Turkey to continue diplomatic talks toward "a joint approach" to Turkey's "legitimate security concerns."

"Unilateral military action into northeast Syria, particularly as U.S. personnel may be present or in the vicinity, while our and local Syrian partner's operations against ISIS holdouts are continuing, is of grave concern. We would find any such actions unacceptable and thus urge Turkey once again to work with us on a joint approach," Ortagus added.

The U.S. has approximately 1,000 troops in Syria still and a small group of diplomats and aid workers who are working to ensure ISIS's "enduring" defeat. To that end, both soldiers and diplomats work closely with the SDF in the areas of northeastern Syria that they and their political counterparts now control.

Within those territories, SDF authorities are holding some 10,000 terror fighters, according to Jeffrey, with 2,000 of them from countries outside Syria and Iraq. The SDF and U.S. have been urging countries to repatriate their citizens for prosecution, with warnings that the local prisons are ad hoc and not a sustainable solution.

There are approximately 70,000 displaced people, who fled ISIS, who are being held in camps, mostly women and children, according to Jeffrey. He said they already face humanitarian issues and potential radicalization.

The Pentagon did not provide details on Monday about its talks with Turkey beyond that the U.S. delegation is led by Brig. Gen. Scott Benedict, the Joint Staff deputy director of political-military affairs for the Middle East.

ABC News' Matt McGarry contributed to this report.