A bomb blast in a four-storey building in the town of Afrin in northwest Syria killed seven civilians and four Free Syrian Army members overnight, state-run Anadolu news agency reported on Monday.
The explosion hit after the Syrian rebels swept into the town with Turkish forces on Sunday, declaring full control after an eight-week campaign to drive out the YPG/PKK terrorists.
Anadolu said the bomb, which it described as planted by terrorists, exploded as the Syrian fighters conducted a search. It ripped out a hole four metres (4.37 yards) deep and badly damaged other buildings and vehicles, it said.
A number of nearby buildings, as well as cars, were also damaged in the explosion, the source said.
Nasir Fuad, an Afrin resident who lost two of his nephews in the blast, told Anadolu Agency that YPG/PKK have targeted people who had refused to extend support to the terror group.
"We were happy because they had left but they did not leave us alone even when they are gone," Fuad said.
An FSA fighter, Fadil Randav, said: "Our brothers were searching for terrorists in the building.
"When they entered [the building] the explosion occurred. The only aim of the terrorists is to kill people. May Allah accept the martyrdom of our friends," he said.
A spokesman for the Turkey-backed Free Syrian Army rebels said on Sunday they had entered Afrin before dawn, meeting no resistance. A war monitor, the Syrian Observatory for Human Rights, said some YPG terrorists defied orders to withdraw, but Turkish forces were in control.
Turkey says the YPG is an extension of the PKK terror group has been waging a terror campaign against Turkish state, and vowed to crush what it described as a "terror corridor" of YPG-controlled territory along Turkey's southern border with Syria.
It has said its next target would be a Syrian region further east where U.S. forces are stationed alongside the YPG.
Turkey on Jan. 20 launched Operation Olive Branch to remove YPG/PKK and Daesh terrorists from Afrin.
According to the Turkish General Staff, Operation Olive Branch has aimed to establish security and stability along Turkey's borders and the region as well as protect Syrians from terrorist cruelty and oppression.
The operation is being carried out under the framework of Turkey's rights based on international law, UN Security Council resolutions, its self-defense rights under the UN charter, and respect for Syria's territorial integrity, it said.
The military has also said that only terrorist targets are being destroyed and "utmost care" is being taken to avoid harming any civilians.
Afrin has been a major hideout for the YPG/PKK since July 2012, when the Assad regime in Syria left the city to the terror group without a fight.
