US officials have repeatedly said their battle against IS in eastern Syria's Deir el-Zour has been complicated by the close proximity of the Syrian regime, backed by Russian Federation and Iranian-linked Shiite militias.
The Britain-based Syrian Observatory for Human Rights said it was not clear if USA -led coalition or Iraqi warplanes carried out the attack Tuesday in the Hassakeh province.
"The United States, the Global Coalition, and local partners, including the Syrian Democratic Forces, are launching operations to liberate the final ISIS strongholds in Syria", the American government said in a statement on Tuesday.
Rights groups said that the vast quantities of ordnance dropped since the start of Operation Shader against IS in 2014 seriously undermined the claim by ministers that the RAF has not caused any civilian casualties in the three-year-long bombing campaign.
In a press conference held in eastern Deir al-Zor, a statement was read by top SDF commanders, which the Kurdish People's Protection Units (YPG) is the leading element, confirming they are fighting in the final assault.
"We welcome the support of Iraqi forces across the border and our partners in the global coalition", he said.
US Defense Secretary Jim Mattis said last week that he expected a "re-energised" effort soon against the ultra-hardline militants in eastern Syria.
"The fight will be hard, but we and our partners will prevail", - said in a statement.
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Deir Ezzor Military Council head Ahmed Abu Khawla told AFP there was a "shared operations room with Iraqi forces".
It was not immediately clear whether the USA -led coalition or Iraqi forces were behind the strike.
The SDF had redeployed hundreds of its forces to western Syria after Turkish troops attacked the Kurdish-held Afrin enclave earlier this year, effectively putting operations against IS on hold.
This resulted in Kurdish forces redirecting their resources from combating ISIS to bolstering fellow Kurds in Afrin.
The SDF on Tuesday announced the launch of a military operation, code-named al-Jazeera Storm, against IS remnants who have reportedly regrouped in the border region between Syria and Iraq after losing control over major populated cities, including its once self-proclaimed capital, Raqqa.
"It had a very big and wide impact on the liberation, which was stalled for months".
The United States will also work with Turkey, Israel, Jordan, Iraq and Lebanon to secure their borders from ISIS and will seek further contributions from regional partners and allies to stabilize liberated territories, the State Department said.
But ISIS has since lost much of that territory to various offensives in both countries.