The US has sent 400 additional troops to Syria to support an allied local force aiming to capture the so-called Islamic State stronghold of Raqqa.
They include Marines, who arrived in the past few days to set up an outpost from which they will be able to fire artillery at IS positions in the city.
Several hundred US special forces soldiers are already deployed, advising Kurdish-led Syrian Democratic Forces.
Coalition air strikes reportedly killed 20 civilians near Raqqa on Thursday.
Several children were among the dead in the IS-held village of Matab, activists say.
The alliance is expected to launch an assault on Raqqa in the coming weeks.
Defence officials told the Washington Post that the Marine artillery unit would have M777 howitzers, which can fire 155mm shells about 32km (20 miles).
A spokesman for the US-led coalition against IS, Col John Dorrian, told Reuters news agency they would help “expedite the defeat” of IS in Raqqa.