The coalition was not immediately available for comment, but has repeatedly said it does its utmost to avoid targeting civilians.
The US-backed Syrian Democratic Forces will begin their final showdown with ISIS in Syria Saturday, an SDF official told Reuters.
One big question has been whether some USA forces in Syria might move to neighboring Iraq, where the United States has more than 5,000 troops helping Baghdad fight ISIS and prevent the group's resurgence.
Success against the Islamic State in Baghouz would also increase pressure on the US military to pull out of Syria, in accordance with Trump's instructions that the troops should leave once the Islamic State has been defeated.
About 20,000 civilians have been evacuated from the area in the eastern province of Deir-el-Zour over the past few weeks to clear the way for a holds barred onslaught.
The source said both Turkey and the USA agreed that the process has gathered momentum.
Most of the dozen or so attackers were killed and two managed to escape after a battle lasting several hours, during which USA airstrikes were called in, according to the Syrian Observatory for Human Rights monitoring group.
At its peak, ISIS held large swaths of Iraq and Syria - an expanse about the size of Portugal.More news: Heat deal two for Suns' Ryan Anderson
Bali told AFP that around 600 IS fighters are still inside the pocket, a lot of them die-hard foreign fighers.
"That is, if a buffer zone that will safeguard terrorists is envisaged or desired, we are against that", said Cavusoglu, referring to United States demands to ensure protection of Kurdish fighters from the People's Protection Units (YPG) who are allied with Washington in the war against ISIS. Manbij and its surrounding areas have remained under the control of the USA -backed Syrian Democratic Forces (SDF) since August 2016.
More than 37,000 people, mostly wives and children of jihadist fighters, have fled IS territory since the SDF intensified its offensive in December, according to the Syrian Observatory for Human Rights.
Some 600 people were able to reach SDF territory on Sunday after fleeing the fighting, the Observatory said.
On Monday, the Observatory said 13 IS militants, including five suicide attackers, were killed as well as six SDF fighters.
The official also warned that the Islamic State would benefit from any USA withdrawal of over 2,000 troops from Syria.
The clash reportedly took place in September in a hamlet IS held in eastern Syria, al-Keshma, near Baghouz.
Islamic State (IS) group leader Abu Bakr al-Baghdadi overcame an attempted coup in a two-day battle with his foreign bodyguards, who were then rounded up and executed, witnesses told the Guardian. Since 2011, Syria has been embroiled in a violent civil war, which created a safe haven for extremists and allowed ISIS to spread quickly.