U.S.-led strikes in Syria have killed 553 people, the Syrian Observatory for Human Rights reported on Thursday.

The Observatory, which relies on reports from activists on the ground, has been providing death-toll estimates since the protests in the country spiraled into civil war beginning in the spring of 2011.

In a statement, the Observatory said 464 of those killed were fighters from the self-proclaimed Islamic State, 57 were fighters with al-Nusra Front and 32 of them were civilians.

"We in SOHR believe that the real number of casualties in the IS is larger than the number which we could document, due to the extreme secrecy by ISIS on its casualties and the difficulties to reach to thus areas," the Observatory said.

If you remember, the U.S. and its allies launched airstrikes against the Sunni extremist group inside Syria back in September. As we told you:

"The Islamic State came to international prominence over the summer, when it began a brazen and lightning-fast attack on Iraq. Since then, the militant group has overtaken several Iraqi cities and has taken responsibility for the beheading of two American journalists."As the Islamic State, which is also known as ISIS and ISIL, moved farther into Iraq, the United States began an air campaign against the group."Copyright 2016 NPR. To see more, visit http://www.npr.org/.