Eight displaced Syrians, including four children, were killed by Turkish border guards while trying to flee their war-torn country Sunday morning, the Syrian Observatory for Human Rights said.
The National Coalition umbrella opposition group – based in Istanbul and backed by Turkey – gave a higher toll of 11 killed and condemned the "terrible tragedy."
Turkey has repeatedly denied accusations of its security forces firing on Syrians trying to cross the frontier, which has been closed for several months.
But the Observatory has recorded numerous incidents this year, and said Sunday's toll was among the highest.
The Britain-based Observatory said the group of eight had been displaced by fighting around the northern Syrian town of Manbij, held by the Islamic State group and under attack by US-backed forces.
They fled northwest towards Idlib province and the border, but were killed "when Turkish border guards opened fire on them as they tried to cross into Turkey," said the Observatory.
It said that four of those killed were children, adding that eight people were wounded, some of them critically.
Since the beginning of the year, at least 60 people – all civilians – have been killed in fire by Turkish border guards, the Observatory said.
In May, Human Rights Watch accused Turkish border guards of shooting and beating Syrian asylum seekers.
Turkish President Recep Tayyip Erdogan denied the reports, and the Turkish army says it only fires at armed smugglers, not at civilians.
But in its statement, the National Coalition expressed its "surprise and condemnation after this terrible tragedy against our brothers fleeing the regime.
"The deaths of defenseless Syrians contradicts the hospitality of the Turkish government and the Turkish people," it said.
Turkey is hosting over 2.7 million refugees from the conflict in Syria. Only a quarter of a million live in refugee camps, while the rest are in towns and cities.
More than 280,000 people have been killed since Syria's conflict erupted in March 2011 with demonstrations against President Bashar al-Assad's government.
AFP contributed this report.