SDF fighters have been on the offensive since November under the cover of airstrikes by the US -led coalition with the aim of eventually surrounding Raqqa and storming it.
Cihan Sheikh Ehmed, spokeswoman for the Syrian Democratic Forces, said the fighting was continuing at the entrance of the Tishrin Dam on the Euphrates river, adding that there are casualties among IS fighters. She gave no further details.
The Kurdish-led Syrian Democratic Forces have ratcheted up the pressure on IS in Syria since the US airlifted hundreds of SDF fighters, USA military advisers, and US artillery behind IS lines on March 21.
The UK-based Syrian Observatory for Human Rights said coalition airstrikes had killed 89 civilians in the countryside west of Raqqa in the past week, including 35 in the Badya school, in the village of al-Mansoura. He did not elaborate.
"To that end, the Coalition continues to prosecute a military campaign against legitimate military targets in support of our partners in Iraq and Syria".
The extremist group has controlled Raqqa since 2014, and it's considered to be IS militants' de-facto capital. Earlier this month, a USA official told reporters that "Raqqa will probably not be the final battle against ISIS".
Fighters from the Syrian Democratic Forces, a US-backed Kurdish-Arab alliance, on Sunday seized a military airport from the Islamic State jihadist group in northern Syria, a spokesman said.
Fears that a nearby dam might collapse are reportedly causing panic and confusion in the Syrian city of Raqqa, a stronghold of the so-called Islamic State (IS) group.
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The Britain- based Syrian Observatory for Human Rights said SDF fighters are clashing inside the village of Karama.
The attacks on the Tabqa and Karama were ongoing under the cover of airstrikes by the US -led coalition, according to the activists.
Mr al-Ja'afari was s peaking at the start of a new round of UN-mediated peace talks.
The two sides are still not meeting face-to-face, despite jointly attending a fourth-round ceremony last month in a United Nations assembly hall.
Another spokesman said the liberation of Raqqa could still take "several months". The French defense minister Jean-Yves Le Drian claimed that the push into Raqqa would begin within days: "France has always said that Raqqa was a major objective, Today, one can say that Raqqa is encircled, that the battle for Raqqa will start in the coming days".
The push toward Raqqa came just a couple of days after US forces were airdropped on Kurdish-controlled areas in northern Syria near Raqqa to aid the SDF in their push toward the city. "This will be a very hard battle but essential", Drian said.
Some 700 US military advisers, Marines, and Rangers have entered Syria to support fighters battling IS militants.