The coalition, as well as US special operations forces, is supporting the Syrian Democratic Forces as they advance in the northern province of Raqa.
The Syrian Observatory for Human Rights said 150 strikes had targeted IS positions around the flashpoint towns of Tal Abyad and Ain Issa since the push was launched on Tuesday.
“There has been a serious intensification of air strikes, but they were most intense on the first day of the operation,” said Observatory head Rami Abdel Rahman.
Abdel Rahman specified that his usage of the word “strike” referred to a missile hitting a target.
The coalition has confirmed carrying out air strikes near Ain Issa and Raqa, but it uses a different definition of “strikes” which makes the tallies difficult to compare.
The Britain-based Observatory also said that clashes in rural territory in northern Raqa province had left at least 31 IS fighters dead since Tuesday.
“There are definitely casualties among the SDF’s ranks but they are not releasing any figures,” he said.
“There are almost no civilians in the villages where the fighting is happening, which is why there is no civilian death toll,” Abdel Rahman told AFP.
But further south in the provincial capital of Raqa city, IS was blocking desperate civilians from fleeing.
“IS is not issuing passes to leave the city — it has even become difficult for people who are sick and need treatment elsewhere,” Abdel Rahman said.
A handful of families had been able to escape the city and had made it west to Idlib province, controlled by a rebel alliance including IS rival, Al-Nusra Front.
There are an estimated 300,000 people still living in Raqa city, the de facto Syrian capital of IS’s so-called “caliphate”.
Abu Mohammed, founder of the Raqa is Being Slaughtered Silently activist group, said residents were paying smugglers $400 (350 euros) each to try to flee the city.
The SDF has warned that IS is using civilians as “human shields”.