WASHINGTON—The U.S. military has completed its second delivery of ammunition to the newly formed Syrian Arab Coalition, a sign of the strengthening relationship between the U.S. and a group of Arab forces that coordinates with Kurdish fighters battling Islamic State in Syria.

U.S. officials confirmed Sunday that the delivery to Syria had been completed sometime Saturday. Officials wouldn't characterize what was provided, except to say it was a big delivery.

“There was a whole lot of ammunition,” an official said.

The U.S., France and their allies are scrambling to respond to a series of attacks in Paris on Friday that left more than 125 dead. The delivery had been scheduled before the attacks, officials said.

This is the second delivery of ammunition since the U.S. became partners with the Syrian Arab Coalition, an umbrella group of about 5,000 Arab forces that is battling alongside Kurdish fighters, all of whom are aligned against Islamic State in Syria.