The city is a key waypost in the SDF's flagship offensive for Raqa, the Syrian heart of IS's so-called caliphate.
"The attack on Raqa will take place in the beginning of the summer," Syrian Democratic Forces commander Rojda Felat told AFP.
She later specified that it would likely start in June based on "military and tactical considerations."
Felat spoke at a press conference in Tabqa, which the SDF captured from IS on Wednesday along with its nearby dam.
The city is a key waypost in the SDF's flagship offensive for Raqa, the Syrian heart of IS's so-called caliphate, about 55 kilometres (34 miles) further down the Euphrates Valley.
The deputy head of the SDF, Qahraman Hassan, said: "In the beginning of the summer, we will storm and liberate (Raqa) city."
He said the alliance would receive "special weapons and armoured vehicles" to enter the city after President Donald Trump changed US policy to allow arms deliveries to Kurdish as well as Arab SDF fighters.
"I believe this support will arrive soon," he said.