The U.S. has found a reliable partner in the YPG, which has been the main force battling the Islamic State group in Syria. The Kurds are moderate, mostly secular fighters, driven by revolutionary fervor and deep conviction in their cause. They are backed by Arab tribesmen, Assyrian Christian gunmen and members of the rebel faction known as Burkan al-Furat — Arabic for the "Volcano of the Euphrates."
"They are a cohesive, well-trained and highly motivated force," said Mutlu Civiroglu, a Washington-based expert on Kurdish affairs. "They are defending their homes and they know the terrain well, unlike the foreign fighters who have joined the IS."
By contrast, Iraqi government forces and allied Shiite militiamen have been slow in retaking IS-held territory. The Iraqis have also suffered occasional losses.
The Iraqis drove IS fighters from Saddam Hussein's hometown of Tikrit in April, but lost Ramadi, the capital of Anbar province west of Baghdad, last month. It was a repeat of the meltdown government forces suffered when IS militants swept across much of the nation's north and west last summer.
It was unclear whether the Syrian Kurds will try to push farther toward Raqqa. Despite the Kurds' recent gains, the Islamic State militants still have another supply line from Turkey that runs through northwestern Syria to Raqqa.
When cornered in the past, the militants have used coordinated mass suicide car bombings and other scorched-earth tactics. The Observatory reported at least four suicide attacks Tuesday against Kurdish and allied forces in northern Syria. Initial reports said at least three Kurdish and allied fighters were killed.
Syria's official SANA news agency said two suicide bombings struck a roundabout in the northeastern city of Hassakeh, killing one person and wounding 13, including five children.
Also Tuesday, a media arm of the Islamic State group in Iraq posted a video purporting to show the killing of over a dozen men it described as spies by drowning them in a cage, decapitating them with explosives, or firing a rocket-propelled grenade at them.
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