Strikes on Iranian-linked bases in Syria would draw a quick response, an Iranian security spokesperson said on Saturday, after the reported death of 19 people in one of the deadliest exchanges between the US and Iranian-aligned forces in years.
"Any pretext to attack bases created at the request of the Syrian government to deal with terrorism and Islamic State elements in this country will be met with an immediate counter-response," Keyvan Khosravi, spokesperson for Iran's top security body, was quoted as saying by the semi-official news agency Nournews.
Iran says its forces and allied fighters are in Syria at the request of Damascus and sees US forces as occupiers.
The death toll in US airstrikes on pro-Iran installations in eastern Syria has risen to 19 fighters, a Syrian war monitoring group said on Saturday.
The US carried out strikes in eastern Syria in response to a drone attack on Thursday that left one American contractor dead and another wounded along with five US troops. Washington said the attack was of Iranian origin.
The UK-based Syrian Observatory for Human Rights 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 US 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 that the United States would "act forcefully" to protect Americans.
Iran's influence in Syria
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.
Iran's foreign ministry condemned the latest US strikes, accusing US forces of targeting "civilian sites."
"Iran's military advisers have been in Syria at the request of the Syrian government to help this country fight terrorism, and shall remain by Syria's side to help establish peace, stability and lasting security," ministry spokesperson Nasser Kanaani told state media.
Tehran's growing entrenchment in Syria has drawn regular Israeli airstrikes but American aerial raids are rarer. The US has been raising the alarm about Iran's drone program.