At least 15 civilians were killed and many wounded in fresh air strikes by US-led coalition in Syria on Thursday morning.
These civilians were targeted in the coalition jets bombardment in the town of Ghandoura, said the Syrian Observatory for Human Rights.
US Central Command, which oversees US military operations in the Middle East, said it had "initiated an assessment following internal operational reporting that a strike today near Manbij, Syria may have resulted in civilian casualties."
The strikes came a day after the coalition opened a formal investigation to determine whether its air strikes last week near Manbij claimed more than 100 civilian lives including children.
Around 600 Syrians including 136 children have been killed in coalition air strikes since September 2014, according to an Observatory toll.
More than 520 US airstrikes since 21 May have killed between 229 and 425 civilians overall.
After examining "internal and external information", the coalition determined that there was sufficient credible evidence of civilian victims to open a formal inquiry, said spokesman Colonel Chris Garver on Wednesday.
The main Syrian opposition group had urged the US-led coalition to suspend its bombardments following the strikes.
Amnesty International has also called on the coalition to redouble its efforts to prevent civilian deaths and to investigate possible violations of international humanitarian law.
— The Syrian Observer (@observesyria) July 27, 2016
The US has rejected calls by Syrian opposition figures after Tokkhar to halt the bombings for the sake of thousands of civilians trapped in Manbij, as it believes the fall of the city is critical to the overriding objective of capturing Raqqa.
“We take all measures during the targeting process to avoid or minimize civilian casualties or collateral damage and to comply with the principles of the Law of Armed Conflict,” the US authorities pledged.