Pro-Iranian forces in Syria have said that they have a "long arm" to respond to further US strikes on their positions, while an Iranian security spokesperson warned of an "immediate counter-response" following tit-for-tat missile and drone attacks in Syria.
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 US air strikes left 19 people dead, according a Syrian war monitoring group, marking one of the deadliest exchanges between the US and Iranian-aligned forces in years.
While it's not the first time the US and Iran have traded strikes in Syria, the recent attacks threaten to up-end recent efforts to de-escalate tensions across the wider Middle East, whose rival powers have made steps toward détente in recent days after years of turmoil.
"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 Iranian state media.
In a separate statement posted online, the Iranian Advisory Committee in Syria wrote: "We have the capability to respond if our centres and forces in Syria are targeted."
"We will have to retaliate ... it will not be a simple revenge."
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.
Iran says its forces and allied fighters are in Syria at the request of Damascus, and sees US forces there as occupiers.
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 monitoring group's head, Rami Abdel Rahman, could not specify the nationalities of the foreigners.
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 growing entrenchment in Syria
Iran has been a major backer of President Bashar al-Assad during Syria's 12-year conflict.
Iran relies on a network of proxy forces throughout the Mideast to counter the US and Israel, its arch regional enemy.
Iran's proxy militias, including the Lebanese group Hezbollah and pro-Tehran Iraqi groups, hold sway in swathes of eastern, southern and northern Syria and in suburbs around the capital.
The US has had forces in north-east Syria since 2015, when they deployed as part of the fight against the Islamic State group, and maintains some 900 troops there, working with Kurdish-led forces that control around a third of Syria.
Tehran's growing entrenchment in Syria has drawn regular Israeli air strikes but American aerial raids are more rare.
The exchange of strikes came as Saudi Arabia and Iran have been working toward reopening embassies in each other's countries. The kingdom also acknowledged efforts to reopen a Saudi embassy in Syria.
The State Department said in a statement on Saturday that they have been in contact with Saudi officials about their approach to the region, including with Syria, adding that Washington's stance against normalisation with Assad's government "remains unchanged, and we have been clear about this."
The State Department said the US's consistent message to regional partners who are engaging with the Syrian government has been that "credible steps to improve the situation for the Syrian people should be front and centre in any engagement."
ABC/wires
