The Syrian Democratic Forces broke into the "Islamic State of Iraq and Syria" (ISIS) group's Syrian bastion of al-Raqa on Tuesday (June 6th), less than an hour after declaring a new phase in their fight for the city.

"Our forces entered the city of al-Raqa from the eastern district of al-Meshleb," SDF commander Rojda Felat told AFP.

She said fierce clashes were also raging on the northern outskirts of the city.

The Syrian Observatory for Human Rights said that the SDF had seized a number of positions inside al-Raqa on Tuesday after advancing to the city's eastern edge overnight.

"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 airstrikes by the international coalition, he said.

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 al-Raqa.

"We declare today the start of the great battle to liberate the city of al-Raqa, 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 al-Raqa from ISIS," Sello said.

He asked civilians inside the city to keep away from ISIS positions and from the front lines.