At least eight pro-Iran fighters were killed in U.S. strikes on eastern Syria, a war monitor said Monday, a day after Washington carried out raids in response to attacks on American forces.

The toll is "eight pro-Iran fighters dead, including at least one Syrian, and Iraqi nationals," the Syrian Observatory for Human Rights said, following the strikes late Sunday on the Mayadeen and Albu Kamal areas of Syria's eastern Deir el-Zour province near the Iraqi border.

U.S. Defense Secretary Lloyd Austin said Sunday the United States had carried out strikes against two Iran-linked sites in Syria in response to attacks on American forces.

It was the third time in less than three weeks that the U.S. military has targeted locations in Syria it said were tied to Iran, which reportedly backs various armed groups that Washington blames for a spike in attacks on its forces in the Middle East.

The surge is linked to Israel's indiscriminate war in Gaza in response to the Oct. 7 incursion by the Palestinian resistance group Hamas.

Israel's military campaign in Gaza since has killed more than 11,100 people, mostly women and children.

About 1,200 people were killed in the Hamas incursion.

"US military forces conducted precision strikes today on facilities in eastern Syria used by Iran's Islamic Revolutionary Guard Corps (IRGC) and Iran-affiliated groups in response to continued attacks against U.S. personnel in Iraq and Syria," Austin said in a statement.

"The strikes were conducted against a training facility and a safe house near the cities of Albu Kamal and Mayadeen, respectively," he added.

Iran's foreign ministry spokesman Nasser Kanani said Monday that "the Americans make unproven claims such as resistance forces taking orders from Iran."

"We have stated many times that the resistance groups do not take orders from Iran, nor do we give them instructions," he said.

"From the very beginning, the Islamic Republic of Iran has repeatedly expressed its concern about the expansion of the scope of the war."