A spokesman for the YPG could not immediately be reached for comment, but had earlier reported an attack by large numbers of ISIS fighters on YPG positions in the area about 50 km (30 miles) north of Raqqa city.
U.S. officials said the air strikes in Raqqa city over the weekend were some of the most intense to date against ISIS in Syria.
The Observatory said the attacks destroyed seven bridges over waterways in the city, which is bordered to the south by the Euphrates river, in a bid to curtail ISIS`s ability to move fighters and equipment.
Ain Issa, the town reportedly recaptured Monday, sits on a major east-west highway that runs all the way from Aleppo in the west to the Iraqi city of Mosul.
The YPG-led forces captured Ain Issa on June 23, part of an offensive that drove deep into ISIS`s stronghold of Raqqa province. The YPG also captured the town of Tel Abyad at the Turkish border in the offensive.
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