Washington (CNN) As President Donald Trump prepares to welcome Turkey's President Recep Tayyip Erdogan to the White House on Wednesday , two US defense officials tell CNN that the US military is in possession of at least one drone surveillance video which the US believes captured a possible war crime being perpetrated by Turkish-commanded fighters in Syria.

The existence of the video was first reported by The Wall Street Journal.

One of the officials cautioned that while the video appears to show a possible extra-judicial execution of a detainee by Turkish-affiliated fighters, they said it does not represent conclusive proof of Turkey's culpability in a war crime, in part because of the quality of the video and absence of additional information about the circumstances being shown.

US officials have repeatedly said that it's possible Turkish-backed fighters in Syria carried out possible war crimes as part of Ankara's incursion aimed at targeting America's Kurdish partners in the fight against ISIS.

One senior official described the Turkish proxy fighters as "thugs" and "bandits" and a US defense official told CNN last week that several pieces of information on potential war crimes weeks have come to the attention of US authorities in recent weeks.

A senior State Department official provided reporters Tuesday with additional details about some of the incidents carried out by Turkish supported opposition forces that the US government is looking into as potential war crimes.

One of those is the October 12 shooting of a Kurdish civilian and politician, Hevrin Khalaf , in her vehicle during the opening days of Turkey's Syrian incursion, the official said.

Her death sparked outrage among US officials as she was seen as a major positive influence in northeast Syria with regard to peace making and reconciliation.

"There was another incident that involved an individual who was filmed by members of one of these TSOs being executed as the individual was on the ground with his hands tied behind him," the official said, using a US government acronym to refer to the Turkish proxy forces.

"There was one report of chemical weapons, specifically white phosphorous being used," the official said, adding that they had seen "several examples of people, medical units and such, being hit by shell fire and such."

The official said the number of instances has not been growing and reiterated that it not a "large" number of incidents.

"We're asking the Turks to chase them down. What the Turks told us is that they do take them seriously," the official said. They added that the Turks told them that they have passed the ones committed by the Syrian National Army to the Syrian National Army's Defense Minister and that "they have set up a commission." The "Syrian National Army" is the name Ankara has given its Syrian proxy force.

"We don't know these people particularly well. We don't know how well they're going to do. As far we're concerned, our interlocutor on these things is Turkey because Turkey has been supporting these people and Turkey took the initiative to go across the border," the official said.

A Pentagon spokesperson said they wouldn't "comment on on purported internal or intelligence reports."

"We remain concerned by reports that Turkish-Supported Opposition forces may have engaged in violations of the law of armed conflict in northeast Syria. We requested additional information from the Government of Turkey, given their support to these groups, and have requested a firm Turkish commitment to prevent violations of the law of armed conflict," they added.

"The Government of Turkey has informed US officials that they take these allegations seriously and are investigating the circumstances surrounding these actions. The United States has urged Turkey to pursue this matter with urgency and to be transparent in any investigation," the spokesperson said.

According to a State Department official, the special envoy for Syria and the Global Coalition to Defeat ISIS Jim Jeffrey discussed "investigating credible reports of violations of the law of armed conflict" in meetings with senior Turkish officials in Ankara last week.

Erdogan invite 'a shame on the United States'

On Tuesday afternoon, Democratic Maryland Sen. Chris Van Hollen called Trump's invitation to Erdogan "a shame on the United States."

"Erdogan is using Jihadi proxies that include a lot of al Qaeda elements and they are committing gross human rights abuses, including that the Trump Administration has acknowledged are war crimes," he said on the Senate floor.

Last week, State Department spokeswoman Morgan Ortagus told CNN in a statement that "in the initial days" of Turkey's offensive into Syria, "we had serious concerns regarding reports that the Turkish-Supported Opposition may have engaged in violations of the law of armed conflict in northeast Syria, including reports of the killing of unarmed civilians and prisoners and reports of ethnic cleansing," adding that "those concerns remain."

Last month, Secretary of Defense Mark Esper said that Turkey and its allies may be liable for war crimes in Syria. "I've seen the reports as well, we're trying to monitor them, they are horrible, and if accurate -- and I assume they are accurate -- they would be war crimes," Esper told CNN's Christiane Amanpour during an exclusive interview at Prince Sultan Air Base in Saudi Arabia.

Last week Defense Department spokesman Jonathan Hoffman told reporters at the Pentagon: "We are aware of -- of allegations of alleged war crimes taking place at the hands of -- of individuals operating in Syria. Any information that we come into about that, we share that with the authorities that -- that should be held accountable for it."

"And so what we've done and will continue to do, and the secretary's raised this issue with his counterparts, is reaching out to the Turks, we'll provide them with information that we have about what we believe is taking place; and we expect them to investigate it, we expect them to hold those people to account, and we'll continue to push that with them," he added.