The Kurds are trying to repel a major offensive which the extremist group launched yesterday against villages in the northern provinces of Raqa and Hasakeh.
The Syrian Observatory for Human Rights said about 80 IS fighters had been killed since Sunday morning in the fighting and US-led air strikes.
Obama said last night the coalition fighting the jihadist group -- also known as ISIL -- would step up its campaign in Syria, while cautioning a long battle remained.
"This is a long-term campaign," the US president said.
Obama said more than 5,000 air strikes had been carried out against the group, eliminating "thousands of fighters, including senior ISIL commanders".
In recent days, the coalition has bombarded IS in a series of heavy raids, particularly targeting its de facto Syrian capital Raqa.
US Defence Secretary Ashton Carter said the raids were intended to help Kurdish forces, who have been a leading ground force partner for the American-led coalition in Syria.
And in recent weeks, they recaptured the key town of Tal Abyad, depriving IS of a conduit through which it transported weapons and fighters.
But IS has fought back with its offensive in parts of Raqa and Hasakeh provinces.
The Observatory said the jihadists took the town of Ain Issa, 55 kilometres from Raqa, but Kurdish officials and activists said the extremists had been repelled.
The Kurds also reclaimed more than 10 villages in Raqa and Hasakeh that were briefly overrun during the IS offensive, the Observatory said.
Elsewhere, the Observatory said six children were among at least nine people killed in regime strikes on the Naseeb area in the southern province of Daraa.
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