The operation coincides with a vast US-backed offensive to retake Iraq's second city of Mosul from the jihadists. (Representative image) Photograph: (AFP)
Washington said it was sending 200 troops to join the 300 it has already deployed to support the Raqa campaign
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.
In a separate anti-IS assault, the Turkish army and its allies on Saturday entered the jihadists` northern Syrian stronghold of Al-Bab, a monitor said.
But in central Syria, IS re-entered Palmyra months after being expelled from the ancient desert city and has retaken most of it, the Syrian Observatory for Human Rights monitor added.
The Syrian Democratic Forces will "begin phase two of the campaign, which aims to liberate territory west of Raqa and isolate the city," SDF spokeswoman Jihan Sheikh Ahmed told reporters.
US Defence Secretary Ashton Carter earlier told a security forum in Bahrain that Washington was sending the 200 troops to join the 300 it has already deployed to support the Raqa campaign.
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 take part in the offensive "on the front lines" alongside SDF fighters.
The Pentagon chief said earlier the "200 additional US forces in Syria" would include bomb disposal experts and trainers as well as special forces personnel.
The jihadists have used car bombs, booby traps and mines as they battle to defend what remains of the territory they took.
"We`re now helping tens of thousands of local Syrian forces to isolate Raqa," 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.
But neighbouring Turkey has also since August pushed its own unprecedented offensive inside Syria against IS and Kurdish fighters.
On Saturday, Turkish troops entered the last bastion IS had in northern Aleppo province, the Observatory said.
Al-Bab had been under jihadist control since 2014.
On Friday, Turkish media said Ankara was sending 300 commandos to reinforce its campaign inside Syria.
The anti-IS fight has been complicated by Turkey`s deep hostility for the SDF.
Ankara regards the alliance`s most powerful military component, the Kurdish People`s Protection Units (YPG), as an arm of the outlawed Kurdistan Workers Party (PKK) that has waged an insurgency in Turkey for decades.
US defence officials said on Thursday they were brokering talks between Turkey and the SDF to prevent any further conflict between them disrupting the anti-IS campaign.
In central Syria, IS fighters on Saturday re-entered Palmyra after regime forces backed by Russia retook the city from them in March.
"After a quick advance, IS now controls most of the city of Palmyra except the southern part," the Observatory`s Rami Abdel Rahman said.
"The airport which is outside the city (just to the east) is now surrounded by jihadists."
He said Syrian and Russia warplanes were in action, "but the problem is that there are not enough Syrian forces in the city".
The Observatory said at least 100 members of pro-government force members have been killed by IS in and around Palmyra since Thursday.
IS caused extensive damage to many of Palmyra`s UNESCO-listed ancient sites after first seizing it in March last year.
Also on Saturday, the Pentagon said the US-led coalition killed a key IS leader in Syria last month.
"Coalition warplanes targeted and killed Tunisian Boubaker al-Hakim, in Raqa, Syria" on November 26, Pentagon spokesman Ben Sakrisson said.
The battle for Raqa coincides with a vast US-backed offensive to retake Iraq`s second city of Mosul from the jihadists.