Yildirim said Turkey can prove that weapons the us supplied to Kurdish allies have been used against Turkish forces by Kurdish separatists inside Turkey.
The US wants Raqqa captured and its dispatch of the Marines to man M777 155mm artillery that will range on the ISIS fighters and pummel them, as it did in the Mosul offensive in Iraq, is key to the strategy.
In his testimony, however, he praised Turkey for clearing Daesh out of large stretches of northern Syria, yet also blamed Ankara for utilising language that he deemed to "have the potential to impact campaign momentum" against Daesh, specifically mentioning recent clashes with Kurdish forces near Manbij.
Despite being a temporary deployment, it could indicate that the White House is giving the Pentagon greater flexibility to make routine combat decisions in the fight against the Islamic State of Iraq and the Levant (ISIL, or ISIS).
That's a big change from the "train, advise and assist" role USA forces have been playing so far - although as with many previous troop deployments to Iraq and Syria, it was not debated, let alone authorized, by Congress.
Syria has called on the United Nations and the Security Council to oblige Turkey to immediately withdraw its "invading forces" from the Syrian territory, stop cross-border attacks and maintain regional and worldwide security and stability. They are preparing to move in to assault the city of Raqqa, IS's self-declared capital, according to reports.
But McCain questioned whether the Trump administration recognized the seriousness of the situation, or how important Turkish cooperation is to U.S. efforts to retake Raqa.
U.S. Lt. Gen. Stephen Townsend, of the anti-IS coalition, said all the forces acting in Syria have converged "within hand-grenade range of one another".
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The SDF spokeswoman said their numbers are increasing with more residents of newly liberated areas from IS joining the ethnically-mixed force, which has been the most effective group on the ground in Syria in the battle against IS.
Instead what has happened is that the SDF have grown closer to the U.S., the Syrian regime and Russian Federation, and the USA finds itself in an awkward position where it may help re-take Raqqa only to see Raqqa eventually handed over to the Syrian government of Bashar al-Assad.
Turkey achieved a milestone in its goals in Syria: It established a foothold in the heart of the country's north after driving Islamic State militants away from its borders and seizing al-Bab, one of the extremist group's major strongholds and a major supply route.
The newly deployed USA troops have been sent to the region to help "whoever is going to take Raqqa", a group that has yet to be determined, the official said.
Kaine noted that Iraq requested USA assistance in ridding the country of ISIS: "But I guess the bottom line is, there is no such request from Syria; we don't judge that - that government capable of making such requests".
Sen. John McCain, an Arizona Republican and the committee chairman, warned of the potential for a "train wreck" at Manbij if the Turks carry out repeated threats to attack Syrian Kurdish fighters backed by the U.S.
The Pentagon is sending about 400 Marines to Syria to help local fighters wrest control of Raqqa, which ISIS considers its capital.
Maintaining peace between the groups in Manbij will be crucial if they will work together in the battle for Raqqa.