The US has sent several hundred Marines to Syria to support an allied local force aiming to capture the Islamic State stronghold of Raqqa.
They reportedly arrived in the past few days to establish an outpost from which they will be able to fire artillery at IS positions some 32km (20 miles) away.
US special forces are already on the ground, advising the Kurdish-led Syrian Democratic Forces (SDF) alliance.
The alliance is expected to launch an assault on Raqqa in the coming weeks.
Over the weekend, a separate force of elite US army Rangers was also deployed near a town north-west of Raqqa in heavily armoured vehicles, in an attempt to end clashes between SDF fighters and a Turkish-backed rebel force.
What are the marines going to do?
The defence officials told the Washington Post that the Marines were from the San Diego-based 11th Marine Expeditionary Unit, and that they had flown to northern Syria via Djibouti and Kuwait.
They are to set up an artillery battery that could fire powerful 155mm shells from M777 howitzers, the officials said.
A spokesman for the US-led multinational coalition against IS, Colonel John Dorrian, told Reuters news agency on Thursday that the artillery battery near Raqqa would help “expedite the defeat” of IS there.
“We have had what I would describe as a pretty relentless air campaign to destroy enemy capabilities and to kill enemy fighters in that area already. That is something that we are going to continue and intensify with this new capability,” he said.
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 Arab and Kurdish fighters. However, their numbers were limited to 500.
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.
Last week, after Turkey’s president said the rebels aimed to capture Manbij, the council said it had agreed a deal with Russia to hand a string of villages on the frontline over to Syrian government forces in order to protect them.
Turkey considers the Kurdish Popular Protection Units (YPG) militia, which dominates the SDF, an extension of the banned Kurdistan Workers Party (PKK), which operates inside 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.
Reports say the review may lead to an increase in the number of US troops in Syria, but not a dramatic shift in strategy.
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 deployed to fight IS in Syria and Iraq if necessary.
In total, about 6,000 US troops are in the countries, but largely in advisory roles.
How is the campaign to capture Raqqa going?
Col Dorrian said the SDF’s current operation to encircle the city, which began in November, was going “very, very well” and could 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, which they have controlled fully since early 2014 and is the de facto capital of the “caliphate” they proclaimed later that year.
IS conflict: US sends Marines to support Raqqa assault