The Syrian Observatory for Human Rights monitor says May 15 coalition air strikes killed 62 people, including 42 civilians.
But it said it was assessing reports of civilian casualties in a strike it launched in Syria's Raqa province.
The Syrian Observatory for Human Rights monitor says May 15 coalition air strikes on Albu Kamal in Deir Ezzor province killed 62 people, including 42 civilians.
The Britain-based monitor had previously said 30 people were killed in the strikes, but subsequently revised the toll upwards.
It said the dead in the town on the Syria-Iraq border included 11 children and 20 IS fighters.
In a statement to AFP, the US-led coalition said its only strikes in the area on May 14 and 15 were carried out outside Albu Kamal.
"The open-source reports of civilian casualties referred to locations inside the city itself, where we did not conduct strikes during the time period of alleged civilian casualties," the coalition said.
"There were other non-Coalition nations conducting strikes in Abu Kamal on the 14th and morning of the 15th though," the statement added, without specifying which countries it was referring to.
Both Syria's government and its Russian ally have carried out strikes against IS in Deir Ezzor, and in February, Iraq's air force carried out strikes against IS fighters in the province for the first time, including on Albu Kamal.
The coalition added that it had carried out strikes on the village of Al-Akeryshi in Raqa province on May 14 and had "forwarded the allegations of civilian casualties to our team for assessment".
The Observatory says those strikes killed 14 people, including 12 women who were farmworkers travelling home in a convoy from the fields.
The US military said in May that coalition strikes in Syria and Iraq had "unintentionally" killed 352 civilians since it launched operations against IS in 2014.
But rights groups say the true number is significantly higher.