Its regional spokesman David Swanson said as many as 100,000 people "may stay trapped" inside during the assault.
One of the strikes hit an internet cafe in the western suburb of Jazra where most of the dead were found, said Rami Abdel Rahman, head of the Britain-based monitor.
The Syrian Democratic Forces militia has spent seven months advancing on the city, with backing from the US-led coalition bombing Daesh in Syria and neighboring Iraq.
Officials were quick to insist that the special forces in Raqqa are "not kicking down doors", but are positioned in the city primarily to advise Kurdish fighters in how to kick those doors down, though they obviously are armed and ready to fight. "It raises a question why they do it and what their real goals are".
The SDF said they captured the majority of Mashlab and also stormed parts of neighborhoods on the western rim of Raqqa amid heavy US strikes backing the SDF in their push to take the city.
"However, in the course of their offensive, the government troops were confronted with counteraction from the aviation of the US-led global coalition".
Pentagon spokesman Eric Pahon said "hundreds" of USA military personnel were taking part in the Raqa offensive.
An estimated 300,000 civilians were believed to have been living under ISIS rule in Raqqa, including 80,000 displaced from other parts of Syria.
Iraqi forces free neighborhood from IS in western Mosul
But thousands have fled in recent months, and the United Nations humanitarian office estimates about 160,000 people remain in the city.
The International Rescue Committee said this week it was "deeply concerned for the safety of civilians in Raqa", noting a drop in the numbers leaving the city.
The SDF - an Arab-Kurdish alliance formed in 2015 - launched its campaign to capture Raqa in November and chipped away at IS territory around the city's north, west, and east.
Talal Sillo, a spokesman for the Kurdish-led SDF said Tuesday that the "great battle" had begun.
Along with Mosul in Iraq, Raqa was one of the twin pivots of the self-styled Islamic "caliphate" that IS declared almost three years ago.
In a video posted on its Aamaq news agency, IS also alleged that the USA -led coalition used white phosphorous munitions over Raqqa on Thursday at dusk, when Muslims would breaking their fasts during the holy month of Ramadan.
Pentagon spokesman Navy Captain Jeff Davis said Syrian regime ally Russian Federation helped prevent an escalation in hostilities between the coalition and the pro-regime forces.