At least nine militants were killed in after-midnight US airstrikes on military sites in Syria's eastern province of Deir al-Zour, a war monitor reported Thursday.

The death toll will rise because of the number of critically wounded fighters, due to US warplanes hitting military sites and a weapons depot in Deir al-Zour, according to the Syrian Observatory for Human Rights.

The Pentagon confirmed the airstrikes, saying in a statement that the US military conducted "self-defense airstrikes" on a facility in eastern Syria in response to attacks against US personnel in Iraq and Syria.