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.
Over the weekend, a separate force of elite US Army Rangers was also deployed near a town north-west of Raqqa in heavily-armoured Stryker vehicles, in an attempt to end clashes between SDF fighters and a Turkish-backed rebel force.
Why are the Marines being sent now?
IS can only be defeated in this war if its militants are forced to stand and fight as a conventional army.
Much of its senior military leadership is made up of former Iraqi army commanders from the Saddam Hussein era, and their instinct the last time they faced a defeat on the battlefield, during the US-led invasion in 2003, was to melt away.
They re-emerged as the leaders of militants opposing the US occupation who then joined to form an umbrella grouping which became al-Qaeda in Iraq. After the start of the Syrian civil war this morphed into IS.
What the US Marines will hope to do, working along aside US special forces, is create a net tight enough to kill or capture these men before they get away. That means co-ordinating the assault and making sure the anti-IS forces work together.
They will hope to finally force the men the US military has been fighting for more than a decade into a last stand.
Isn't there a limit on the number of US troops in Syria?
Under President Barack Obama, US special operations forces were deployed to recruit, train and advise the SDF's 30,000 Arab and Kurdish fighters. However, their numbers were limited to 503.
The Marines' deployment is considered temporary, so it is not affected by the cap.
What about the Rangers?
Col Dorrian said the dozens of Rangers who had arrived on the outskirts of Manbij, about 110km (68 miles) from Raqqa, were also there "for a temporary period".
The Rangers were seeking to "create some assurance", Col Dorrian added, following clashes between Turkish-backed Arab rebels and local fighters from the Manbij Military Council, which was set up by the SDF when it captured the town.
The Turkish government considers the Kurdish Popular Protection Units (YPG) militia, which dominates the SDF, a terrorist group because of its links to the Kurdistan Workers Party (PKK), which is banned in Turkey.
Does this still signal an escalation in US involvement?
It is not yet clear but the deployment comes as President Donald Trump considers a new plan to defeat IS that was submitted by the Pentagon late last month.
The Associated Press news agency reports that Mr Trump wants to give the Pentagon greater flexibility to make routine combat decisions in the fight against IS.
Commanders on the ground were frustrated by what they considered micromanagement by the Obama administration, it adds.
The US is also said to be preparing to send up to 1,000 troops to Kuwait to serve as a reserve force that can be deployed to fight IS in Syria and Iraq if necessary.
How is the campaign to capture Raqqa going?
Col Dorrian said the SDF's operation to encircle the city was going "very, very well" and might be completed in a few weeks. "Then the decision to move in can be made," he added.
Earlier this week, the SDF cut the main supply route connecting Raqqa to IS-held territory to the south-east in Deir al-Zour province.
The New York Times meanwhile cited US officials as saying there were an estimated 4,000 militants inside Raqqa.