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. US special forces are already in Syria.
Meanwhile, US-led coalition air strikes killed 20 civilians – including children – near the city, reports say.
US Secretary of State Rex Tillerson is to host talks with coalition members ahead of an expected assault on Raqqa.
Foreign ministers and senior officials from 68 nations and international organisations had been invited to attend a two-day gathering in Washington beginning on 22 March, the state department said.
“Secretary Tillerson has been crystal clear that defeating Isis (IS) is the state department’s top priority in the Middle East,” acting state department spokesman Mark Toner said.
Defence officials told the Washington Post that a Marine artillery unit had been deployed with large field guns that can fire 155mm shells about 32km (20 miles).
A coalition spokesman, 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 vehicles.
The move was an attempt to end clashes between units from the Kurdish-Arab alliance, known as the Syrian Democratic Forces (SDF), and Turkish-backed rebels.
Why are Marines being sent now?
IS can be defeated in this war only if its militants are forced to stand and fight as a conventional army, the BBC’s Paul Danahar writes from Washington.
Much of its senior military leadership is made up of former Iraqi army commanders from the Saddam Hussein era. 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 alongside 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 that the US military has been fighting for more than a decade into a last stand.
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