The U.S. -led coalition said earlier that it carried out Wednesday air strikes against "Syrian pro-regime forces" that launched an "unprovoked attack" on Kurdish-led Syrian Democratic Forces (SDF)' headquarters and killed an estimated 100 militiamen.
"We also can not speculate on the exact make-up of the Syrian pro-regime forces who conducted the attack at this time", Veale said.
Syrian regime forces' initial coordinated attack included tanks, howitzers and more, but only one US-backed SDF fighter was injured.
American forces engaged Pro-Assad troops with artillery, rockets and air strikes on Tuesday following what CENTCOM has characterized as an "unprovoked attack" against USA backed forces in Syria.
Regime forces were likely attempting to take an oilfield near Khusham.
"Regarding the recent event near Khusham, Syria, the Coalition had been observing a slow buildup of Syrian pro-regime forces (PRF) over the past week", Veale pointed out.
The U.S. maintains about 2,000 troops in the country.
Russia's Ministry of Defense indicated Thursday that the pro-regime forces had not been coordinating with Russian officials in the area.
Gayle Tzemach Lemmon interviews members of a women’s cooperative outside the town of Amudah Syria
Those pro-regime vehicles and personnel who retreated were not targeted by coalition air and artillery strikes, he said.
In June a U.S. warplane shoots down a Syrian jet for the first time after it bombs SDF fighters. The Syrian Observatory said those involved and killed by the strikes were tribesmen loyal to President Bashar al-Assad and Shia militias of Afghanistan.
The Syrian Democratic Forces have been a crucial American ally in the fight against ISIS.
Coalition and Russian officials were "in regular communication...before, during and after" the assault, the same official said.
The Syrian Foreign Ministry on Thursday strongly condemned the US-led air strikes, accusing the USA war planes of "committing a massacre" against pro-government forces.
Before the US counter-attack began, American military officials called their Russian counterparts on a pre-established hotline to voice concern about the build-up.
Syria's foreign ministry appealed to the United Nations Security Council Thursday to condemn a US assault that killed more than 100 government troops.
In June past year, a U.S. Navy jet shot down a Syrian plane after it attacked U.S. -backed fighters on the ground.
The Damascus regime and Russia, Mr Assad's main foreign backer, have condemned the attack, with the strikes underlining the risk of a direct confrontation between global players in a multisided war.