U.S. Army Colonel Ryan S. Dillon, a spokesman for the U.S. -backed coalition against Islamic State, said the Syrian Democratic Forces who would lead the offensive had closed a ring around Raqqa, but he did not confirm the assault had started.
The conflict monitor, Syrian Observatory for Human Rights, reported that the SDF, supported by U.S. special forces, were working to clear mines and any remaining militants from the town and dam areas.
Syria's Al-Khabour news channel reported that the PMF had controlled on Thursday morning the two villages of Qusayba and Bawaridi, southeast of Hasaka after a surprising withdrawal of Daesh "ISIS militants" from the area.
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There also are reports of mounting civilian casualties, though they are hard to confirm because of the war environment. Activists reported that the militants were forcing them to stay and using them as human shields.More news: Canadian 'impacted' by London attack
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The deadliest single strike was the attack on Mosul's al-Jadida district on 17 March, where at at least 101 men, women and children were killed. The commanders leading the military campaigns in Afghanistan, Iraq, North Africa, and Syria - all places with significant Islamic State presences - also remain in place.
On Wednesday, US warplanes reportedly bombed a communications center in the heart of Raqqa city, putting all landlines out of service, pro-opposition news site Enab Baladi reported. But U.S. officials have said they're unsure if the forces are Syrian or Iranian, or members of Hezbollah or another militia. The U.S. has also given assurances over Turkey's concerns that the weapons provided to the YPG would not be used against Turkey by terror groups operating inside Turkey, Yıldırım said, "They have told us that they will specifically track [the use of weapons]".
"In each of the incidents, the investigation assessed that all feasible precautions were taken and the decision to strike complied with the law of armed conflict", a spokesperson for CentCom said, adding that civilian casualties had occurred in less than 1 per cent of more than 21,000 strikes over the past three years. In May, SDF fighters captured Tabqa, a previously ISIL-held town some 50km west of Raqqa, and a strategic dam nearby.
Roadblocks and damage to bridges and infrastructure has driven up the prices of fuel and basic foodstuffs inside Raqqa, according to the U.N.'s humanitarian agency OCHA, compounding the hardship inside the extremist group's self-styled capital.
The US has backed the SDF with weapons, air power, and ground support in its campaign to defeat the Islamic State group in Syria.