Airstrikes have killed at least 23 civilians in one of the last pockets of Islamic State-controlled territory in Syria, according to Syrian state media.
Meanwhile, US-backed forces in the area announced they have resumed their campaign against the extremists.
The Britain-based Syrian Observatory for Human Rights said it was not clear if the airstrikes in the Hassakeh province were carried out by the US-led coalition or the Iraqi air force.
It said the strikes killed 10 children, six women and seven elderly people.
The state-run Syrian News Agency said 25 civilians were killed in the airstrikes south of the town of Shadadi, blaming the US-led coalition.
The strikes took place in an area where the US-backed and Kurdish-led Syrian Democratic Forces are battling IS.
Lelwa Abdullah, an SDF spokeswoman in the adjacent Deir el-Zour province, said the final phase of a large operation against IS in eastern Syria has begun.
She said the SDF will "liberate those areas and secure the Syrian-Iraqi border and end the IS presence in eastern Syria once and for all".
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.
Ms Abdullah said IS attacks have increased in recent weeks in parts of eastern Syria near the border with Iraq as the extremist group seeks to regroup.
She said the clearing operations will take place with the help of the US-led coalition and Iraqi forces across the border.
Elsewhere in Syria on Tuesday, more than three dozen Syrians held for years by al-Qaida-linked insurgents in the country's north west were released as part of a deal to hand over areas around Damascus to the government, state media reported.
State-run Al-Ikhbariya TV broadcast images of the released men, women, children, who arrived by bus at a government-controlled checkpoint in Aleppo province.
Many were in tears, and they could be seen kissing and hugging Syrian soldiers. The captives had been held by the insurgents in northern Syria since 2015.
It is the latest in a series of evacuation deals for areas around the capital that have been besieged for years and subjected to heavy bombardment by government forces.
The UN and rights groups have criticised the deals, saying they amount to forced displacement.
The latest deal concerns Yarmouk, a Palestinian refugee camp that was a built-up residential area before the civil war.
IS militants still control parts of the camp and a neighbouring area, where they are battling government forces.
The 42 people freed on Tuesday are the first batch of more than 80 to be released.
Under the deal, fighters from the al Qaeda-linked Hayat Tahrir al-Sham group will withdraw from Yarmouk, while about 5,000 people in Foua and Kfraya, two north-western villages besieged by insurgents, will be allowed to relocate to government-held areas.