A US-backed Arab-Kurdish alliance announced Saturday "phase two" of its campaign for the Islamic State group's Syrian bastion of Raqa as Washington said it was sending 200 more troops to back the offensive.
Syrian Democratic Forces (SDF) officials told AFP that US soldiers would be "on the front lines" of the push for the northern city.
US Defence Secretary Ashton Carter earlier on Saturday told a security forum in Bahrain that Washington was sending the 200 troops to join the 300 it has already deployed to support the campaign.
The SDF will "begin phase two of the campaign, which aims to liberate territory west of Raqa and isolate the city," spokeswoman Jihan Sheikh Ahmed told reporters.
With a pre-war population of about 240,000, Raqa is the de facto capital of the self-styled caliphate IS declared across Iraq and Syria in 2014.
Speaking in the village of Aaliyah, north of Raqa, Ahmed said the SDF had captured 700 square kilometres (270 square miles) of territory since it began its advance on the city on November 5.
The alliance had also grown in size, she said, with more than 1,500 local fighters joining forces with the SDF after being "trained and equipped by the international coalition".
The SDF's coordination with the US-led coalition "will be stronger and more effective during the second phase of the campaign," Ahmed said.
Two SDF officials told AFP that US soldiers would be taking part in the offensive "on the front lines" alongside SDF fighters.
"US forces were on the front lines of the first phase of this offensive, and one member of these forces was killed. Their participation will be even more effective alongside our forces in the second phase," said SDF spokesman Talal Sello.
SDF adviser Nasser Hajj Mansour said: "American forces will be on the front lines of this phase."
The Pentagon chief said the "200 additional US forces in Syria" would include bomb disposal experts and trainers as well as special forces personnel.
The Islamist militants have used car bombs as well as booby traps and mines as they battle to defend what remains of their territory.
"We're now helping tens of thousands of local Syrian forces to isolate Raqa," which has also served as a hub for Islamist militants plotting attacks abroad, Carter said.
Backed by coalition air strikes, the SDF has pushed south from areas near the Turkish border, advancing to within 25 kilometres (15 miles) of the city.
The offensive has been complicated by the deep hostility to the SDF of Turkey, a NATO ally and Syria's neighbour.
Ankara regards the alliance's most powerful military component, the Kurdish People's Protection Units (YPG), as an arm of the outlawed rebel Kurdistan Workers Party (PKK) that has waged a deadly insurgency in southeastern Turkey for three decades.
The SDF controls a large swathe of northeastern Syria along the Turkish border and a smaller enclave in the northwest.
After advances that looked set to link the two areas, the Turkish army entered Syria in August in an operation it said targeted both IS and the YPG.
Turkish troops have since attacked Kurdish forces multiple times even as they have suffered losses at the hands of IS.
US defence officials said on Thursday they were brokering talks between the two sides towards preventing any further conflict between them disrupting the campaign against IS.
Colonel John Dorrian, spokesman for the US-led coalition, said they were "facilitating joint discussions with Turkey, the SDF and other coalition partners to promote deescalation in the area".
The battle for Raqa coincides with a vast US-backed offensive to retake Iraq's second city of Mosul from the Islamist militants.
Raqa and Mosul are the last major urban centres under IS control after the jihadists suffered a string of territorial losses in both countries over the past year.
In Syria, IS still holds Al-Bab, to the northwest of Raqa, and most of the city of Deir Ezzor, to the southeast.