"We don't know for the moment if the US-led global coalition or Iraqi forces carried out the strike" on the village of Al-Qasr in Hasakeh province, the Observatory said.
A civilian was "regrettably" killed just over a month ago by RAF airstrikes targeting Islamic State (IS, formerly ISIS) in Syria, the United Kingdom defense secretary has said in an unprecedented admission.
The US State Department said later that the coalition and its "partners, including the Syrian Democratic Forces, are launching operations to liberate the final ISIS strongholds in Syria".
ISIS swept across large parts of Syria and neighboring Iraq in 2014, declaring a so-called "caliphate" in areas they controlled.
Turkey's offensive to expel the People's Protection Units (YPG) from Afrin - where the United States has no presence - led to a pause in the campaign against ISIS, the Pentagon has said.
Combined Joint Task Force Operation Inherent Resolve, representing 71 nations and four global organizations, is committed to the defeat of ISIS in designated areas in Iraq and Syria and to setting the conditions for follow-on operations to increase regional stability, officials said.
Lelwa Abdullah, an SDF spokeswoman, said on Tuesday that the Kurdish-led force will "liberate those areas and secure the Syrian-Iraqi border and end the IS [ISIS] presence in eastern Syria once and for all". Syria's state-run SANA news agency held the coalition responsible, but the USA -led coalition told the AP that initial reports suggested there were no coalition airstrikes on the area.
Then, the SDF redeployed 1,700 fighters from eastern fronts against IS to help fight the Turkish military, which then captured Afrin in March.
It now appears that a civilian on a motorbike was killed after they entered the target area at the last minute.
Israel has not confirmed or denied responsibility for the strike, but Iran and the Syrian government hold Tel Aviv responsible for Sunday's missile attack in northern Syria.
The UK admission will also place focus on RAF air strikes around Mosul previous year, after a BBC investigation showed that British jets are likely have been responsible for civilian deaths during the intense bombing campaign to liberate the city from Islamic State. He said the time when Tehran's enemies can "hit and run" is over.
"To suggest they have not - as has been done - is nonsense", they added. More than three dozen Syrians held for years by al-Qaida-linked insurgents in the country's northwest were released on Tuesday as part of a deal to hand over areas around Damascus back to the government, state media reported.
The agreement caps a string of similar surrender deals that have seen rebels exiting a number of pockets around Damascus in recent weeks, including the town of Dumayr and the eastern Qalamoun region.