US air strikes against Iran-backed armed groups on the Syrian-Iraqi border killed at least seven fighters overnight into Monday, sparking immediate calls for revenge and fears of a new escalation between Washington and Tehran.
The second such raid on pro-Iran targets since US President Joe Biden took office, described by the Pentagon as "retaliatory", came despite faltering efforts to revive a key deal over Iran's nuclear programme.
US defence spokesman John Kirby said in a statement that three military facilities used by Iran-backed militia had been hit -- two in Syria and one in Iraq.
Kirby said that the targets had been used by "Iran-backed militias that are engaged in unmanned aerial vehicle (UAV) attacks against US personnel and facilities in Iraq."
Kataeb Hezbollah and Kataeb Sayyid al-Shuhada, two Iraqi armed factions with close ties to Tehran, were among the "several Iran-backed militia groups" that had used the facilities, Kirby said.
US interests in Iraq, where 2,500 American troops are deployed as part of an international coalition to fight the jihadist Islamic State group, have been targeted in more than 40 attacks this year.
The vast majority have been bombs against logistics convoys, while 14 were rocket attacks including some claimed by pro-Iran factions hoping to pressure Washington into withdrawing all its troops.
"Given the ongoing series of attacks by Iran-backed groups targeting US interests in Iraq, the president directed further military action to disrupt and deter such attacks," Kirby said.
"Specifically, the US strikes targeted operational and weapons storage facilities at two locations in Syria and one location in Iraq, both of which lie close to the border between those countries," he added.
The Syrian Observatory for Human Rights, which relies on a broad network of sources on the ground, said seven fighters had been killed in the strikes in the early hours of Monday morning local time.
The war monitor also said at least six more fighters were wounded in the raid, among whose targets was an arms depots near Albu Kamal, a Syrian town which lies where the border crosses the Euphrates river.
"A weapons storage facility and a military position near Albu Kamal," Rami Abdel Rahman, the head of the UK-based monitoring organisation, told AFP.
Syria's state-run SANA news agency said one child had been killed in the raid but gave few details.
The Hashed, a paramilitary alliance that includes several Iranian proxies and has become the main power broker in Baghdad, admitted to losses in the strikes and vowed revenge.
The US overnight strikes "resulted in the martyrdom of a group of heroic fighters", it said.
"We will remain the shield defending our beloved nation, and we are fully ready... to respond and take revenge."
Some of the militia groups that form the Hashed al-Shaabi have been deployed in Syria over the years to support regime forces and to further Iran's interests in the country.
In February, US strikes on facilities in east Syria used by Iran-backed militia groups left more than 20 fighters dead, according to the Observatory.
The latest US strikes come two days after the United States and France warned Iran that time was running out to return to a nuclear deal, voicing fears that Tehran's sensitive atomic activities could advance if talks drag on.
A return to the 2015 Iran accord has been a key Biden promise after the nuclear deal was trashed by his predecessor Donald Trump.
"We have a national interest in trying to put the nuclear problem back in the box that it was" under the deal, US Secretary of State Antony Blinken said.
The UN's nuclear watchdog said Friday it had received no reply from Tehran over the possible extension of a temporary agreement covering inspections at Iranian nuclear facilities which expired on Thursday.
Announcement of the strikes came one day before Biden meets at the White House with Reuven Rivlin, president of Israel, Iran's arch foe.
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