Washington (CNN) Secretary of Defense Mark Esper said Friday the US is not abandoning its Kurdish allies who are now under attack from Turkey's military offensive into Syria, even as he made clear the US will not intervene in the fight.

"We are not abandoning our Kurdish partner forces and US troops remain with them in other parts of Syria," the defense secretary said in remarks to reporters at the Pentagon. Esper delivered his remarks shortly before the administration announced it would give the Treasury Department the authority to hit Turkey with significant sanctions -- but wouldn't be using them just yet.

"We remain in close coordination with the Syrian Democratic Forces who helped us destroy the physical caliphate of ISIS, but I will not place American service members in the middle of a longstanding conflict between the Turks and the Kurds, this is not why we are in Syria," Esper said.

The defense secretary said that the US was urging Turkey to forgo its operation and instead address its concerns through the development of a security zone along the border. Previous efforts to establish such a zone, which involved the US convincing the Kurds to dismantle their defensive fortifications and withdraw their fighters from the border with Turkey, collapsed after Ankara launched its incursion.

Esper also said the US would continue to work with the 80 members of the Defeat ISIS coalition and the Kurdish-led Syrian Democratic Forces to ensure the defeat of ISIS, despite the fact that SDF and US officials have told CNN that the SDF has been forced to suspend its counter-ISIS campaign due to the Turkish offensive.

'Sold out'

Chairman of the Joint Chiefs of Staff Gen. Mark Milley told reporters at the Pentagon Friday that the remaining 1,000 US troops in Syria were still relying on the SDF to protect therm.

"We're asking them to continue their partnership with us and continue their -- a lot of it is force protection of our forces...we're encouraging them not to over react," Milley said.

Some of the Kurdish forces guarding a US military base in northern Syria told CNN's Clarissa Ward recently that they feel they've been sold out by their American allies due to US inaction in the face of the Turkish attack.

It's a scenario that has some analysts worried about the safety of US troops.

Small units such as Special Forces are "operating by themselves in dangerous places, surrounded by large quantities of foreign forces and for that to work, for that to be safe for the Americans, we have to earn trust and if others," said Brad Bowman, senior director of the Center on Military and Political Power at Foundation for Defense of Democracies.

"If people come to see the United States as untrustworthy or transactional or fickle, then it's going to be hard to earn that trust and it's going to be a scary thing for US troops who are wondering whether those foreign fighters will turn on them," Bowman said.

'Really not good'

Esper held the Pentagon briefing as lawmakers from both parties, former officials and analysts have excoriated President Donald Trump for pulling US troops back in northeastern Syria and, in doing so, making room for Turkish President Recep Tayyip Erdogan to act on his long-held goal to attack the Kurds fought for and with the US against ISIS.

Republican lawmakers have denounced the move as a betrayal of the Kurds and a strategic blunder that will weaken American credibility, reverse gains against ISIS, make it harder for the US to build alliances and give a boost to Russia , China and Iran.

Privately, members of the military echo similar concerns. Three US military officials CNN spoke with said there is widespread frustration about what is happening to the Kurds in Syria. "How do we expect anyone to partner with us now," said one official. "They did everything we asked them to do" said another. "This is really not good for us."

On Friday, Esper said he had spoken with Turkish Defense Minister Akar the day before. A Pentagon statement about that call said Esper had "strongly encouraged" Turkey to stop the operation so that the US and other partners could "find a common way to deescalate the situation before it becomes irreparable."

Esper also warned Akar that the military offensive risked "serious consequences for Turkey" and that "Turkey's actions could harm US personnel in Syria." The statement made no explicit reference to the Syrian Kurds or the Syrian Democratic Forces.

Potential sanctions

Trump made a gesture Friday at possible "consequences," signing an executive order that Treasury Secretary Stephen Mnuchin said allows for "significant new sanctions authorities that can be targeted at any person associated with the government of Turkey, any portion of the government." Mnuchin said, however, that the US wouldn't be activating the sanctions at this time.

"We hope we don't have to use them, but we could shut down the Turkish economy if we need to," Mnuchin said, adding that Trump is "concerned" about the Turkish military offensive in Syria and the potential of targeting civilians and civilian infrastructure. Treasury said one potential sanctions trigger would be if Turkey undermined "the continued counterterrorism activities of the Syrian Democratic Forces." Mnuchin said Trump wants to make it clear that "it is imperative that Turkey not allow a single ISIS fighter to escape."

On Friday, the SDF reported that five ISIS members had escaped from a prison after Turkish mortars landed nearby.

The shelling happened in the Syrian city of Qamishli, where earlier Friday a car bomb exploded, killing four people and injuring nine. ISIS claimed responsibility for the attack.

The Turks have conducted airstrikes with fixed-wing manned aircraft and unmanned aerial vehicles, they've conducted artillery strikes and some direct fire from tanks on their side of the border, on the Turkish side of the border, Milley said Friday.

Milley added that several hundred Turkish soldiers and about 1,000 Turkish-backed Syrian proxy forces had advanced slightly on the ground. "The incursion on the ground is relatively limited, but the indirect-fire strikes and the airstrikes continue," he said.

A US official familiar with the situation on the ground said that on Thursday, Turkey's Syrian proxy force had moved in around the cities of Ral Abyad and Ras al-Ayn. The SDF and the Turkish proxies "have been fighting, hard to say exactly what gains/losses on either side."

This official said the US was "seeing reports of civilian casualties from what looks like indiscriminate bombing of dense population centers. Tragic imagery of what are clearly civilian men, women and children." The official said that while Turkey's air strikes appeared to be indiscriminate, no strikes had landed near the locations of US forces.

According to another US defense official, the US has shared a list of "no-strike" locations with the government of Turkey, which goes beyond just where US troops are located and includes Kurdish areas, including SDF-controlled prisons holding ISIS prisoners that the US sees as necessary to the fight against the terrorist group.

On Friday, Esper indicated that Turkey's operation had put the Trump administration between a rock and a hard place.

"The impulsive action of President Erdogan to invade northern Syria has put the United States in a tough situation given our relationship with our NATO ally Turkey, who has fought alongside the United States in the past, the Syrian Democratic Forces, who helped us destroy the physical caliphate of ISIS, and the safety of US military personnel," Esper said.

'Greatly disappointed'

"We oppose and are greatly disappointed by Turkey's decision to launch a unilateral military incursion into northern Syria," Esper said at the Pentagon Friday. "This operation puts our SDF partners in harm's way, it risks the security of ISIS prison camps and will further destabilize the region."

Early reports from northeastern Syria appear to confirm concerns that the fighting could undermine gains made against ISIS. Since the fighting began the SDF has suspended operations against the militants.

SDF spokesman Mustafa Bali, speaking Friday after the ISIS prisoner escape, accused Turkey on Friday of attempting to release ISIS prisoners as part of the Turkish military offensive. "There is a real threat for the situation to get out of control," he said in a statement.

On Friday, Esper said that when Turkey notified US officials of their imminent military operation, which began Wednesday, the Pentagon "relocated a small contingent of less than 50 Special Operations Soldiers out of the immediate zone of attack" to ensure American troops were not caught up in the fighting.

Esper stressed that the US will remain focused on the safety of its men and women in uniform and "as such we are re-positioning additional forces in the region to assist with force protection as necessary."