Monitor says the advance came after heavy air strikes by the US-led coalition killed 21 civilians trying to flee
The US-backed Syrian Democratic Forces broke into the Islamic State-held city of Raqqa on Tuesday after declaring a new phase in their fight for the Syrian city.
The drive for the heart of IS's Syrian territory has been seven months in the making and has been backed by US-led coalition air support, military advisers and weapons deliveries.
On Tuesday, Arab and Kurdish SDF fighters advanced to the city's eastern edge and broke into Raqqa for the first time.
'They are fighting street battles inside Raqqa now, and we have experience in urban warfare'
- Rojda Felat, SDF commander
"Our forces entered the city of Raqqa from the eastern district of Al-Meshleb," SDF commander Rojda Felat told AFP.
"They are fighting street battles inside Raqqa now, and we have experience in urban warfare," she said.
Fierce clashes were also raging on the northern outskirts of the city, which lies on the winding Euphrates river, Felat added.
The Syrian Observatory for Human Rights said the SDF had seized a number of positions inside Raqqa.
"They have taken control of a checkpoint in Al-Meshleb, as well as a number of buildings," Observatory head Rami Abdel Rahman said. "The advance came after heavy air strikes by the US-led coalition," he told AFP.
After months sealing off access routes to the city from the east, north and west, the SDF on Tuesday announced a new phase in their fight for Raqqa.
"We declare today the start of the great battle to liberate the city of Raqqa, the so-called capital of terrorism and terrorists," SDF spokesman Talal Sello told reporters in the village of Hazima, north of the city.
"With the international coalition's warplanes and the state-of-the-art weapons they provided to us, we will seize Raqqa from Daesh," Sello told AFP, using an Arabic acronym for IS.
He asked civilians inside the city to keep away from IS positions and from the front lines.
'With the international coalition's warplanes and the state-of-the-art weapons they provided to us, we will seize Raqqa from Daesh'
- Talal Sello, SDF
As the SDF has drawn closer to the city, reports of civilian casualties in coalition air strikes have swelled.
On Monday, the Observatory said a coalition bombing raid killed 21 civilians as they tried to escape Raqqa by dinghy on the Euphrates river.
The same route leading out of the city's southern districts had been used by IS fighters, Abdel Rahman said.
An estimated 300,000 civilians were believed to have been living under IS rule in Raqqa, including 80,000 displaced from other parts of the country.
But thousands have fled in recent months, seeking safety in territory newly captured by the SDF.
More than 320,000 people have been killed since civil war erupted in Syria in 2011.
It began with anti-government protests but has since evolved into a complex multi-front war involving the army and rebel groups as well as the SDF and IS.