The death toll from United States air strikes on pro-Iran installations in eastern Syria has risen to 19 fighters, a Syrian war monitor says, in one of the deadliest exchanges between the US and Iran-aligned forces in years.
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The US carried out strikes in eastern Syria in response to a drone attack on Thursday that left one American contractor dead and another one wounded along with five US troops.
Washington said the attack was of Iranian origin.
The retaliatory strikes by the US on what it said were facilities in Syria used by groups affiliated with Iran's Revolutionary Guard Corps left a total of 19 dead, according to the UK-based Syrian Observatory for Human Rights.
The war monitor said air raids killed three Syrian troops, 11 Syrian fighters in pro-government militias and five non-Syrian fighters who were aligned with the government.
The monitor's head Rami Abdel Rahman could not specify the nationalities of the foreigners.
Reuters was unable to independently confirm the toll.
The initial exchange prompted a string of tit-for-tat strikes.
Another U.S. service member was wounded, according to officials, and local sources said suspected US rocket fire hit more locations in eastern Syria.
President Joe Biden on Friday warned Iran the US would "act forcefully" to protect Americans.
"Make no mistake: the United States does not ... seek conflict with Iran, but be prepared for us to act forcefully to protect our people," he told reporters during a visit to Canada.
Asked whether there should be a higher cost for Iran, Biden replied: "We're not going to stop."
Iran has been a major backer of President Bashar al-Assad during Syria's 12-year conflict.
Iran's proxy militias, including Lebanese group Hezbollah and pro-Tehran Iraqi groups, hold sway in swathes of eastern, southern and northern Syria and in suburbs around the capital.
Tehran's growing entrenchment in Syria has drawn regular Israeli air strikes but American aerial raids are rare.
The US has been raising the alarm about Iran's drone program.
Australian Associated Press