Air strikes in eastern Syria have killed 26 fighters from an Iran-backed Iraqi paramilitary group following a deadly attack on U.S.-led coalition forces in neighboring Iraq.
The British-based Syrian Observatory for Human Rights said the March 12 strikes near the Syrian border town of Albu Kamal were probably carried out by the coalition.
But a spokesman for the coalition said in an statement to AFP that it "did not conduct any strikes in Syria or Iraq last night."
Later in the day, U.S. Defense Secretary Mike Esper blamed Iranian-backed Shi'ite militia groups for the attack on the coalition at the Camp Taji military base, located less than 30 kilometers north of Baghdad.
But he did not confirm whether the U.S. or its allies had carried out the eastern Syria attack.
However, Esper said that "all options are on the table" as Washington and its allies try to bring those responsible for the attack, which killed two U.S. troops and one British soldier and wounded a dozen others when a barrage of Katyusha rockets were launched from a truck later discovered several kilometers from Camp Taji.
Syrian state media reported that in the attack in eastern Syria, unidentified jets hit targets southeast of Albu Kamal with only material damage.
However, the Observatory said camps of the Popular Mobilization Forces, an umbrella grouping of Iran-backed Shi'ite militias, were hit in the strikes, which came after a rocket attack on the Camp Taji military base, located less than 30 kilometers north of Baghdad.
U.S. Secretary of State Mike Pompeo and British Foreign Secretary Dominic Raab "underscored that those responsible for the [Camp Taji] attacks must be held accountable," the State Department said of a phone call between the two.
Iraq's military said caretaker Prime Minister Adel Abdul-Mahdi ordered an investigation into what he called “a very serious security challenge and hostile act."
No-one claimed responsibility for the rocket attack, but the United States has accused Iran-backed militias of previous attacks on Iraqi bases hosting coalition forces.
U.S. Marine General Kenneth McKenzie, the head of Central Command, told a Senate hearing that the attack was being investigated.
But he noted that Iran-backed Kataib Hezbollah "the only group known to have previously conducted an indirect fire attack of this scale against U.S. civilian and coalition forces in such an incident Iraq."
U.S. President Donald Trump on March 12 said it had not been fully determined whether Iran, which has backed a number of anti-U.S. militia groups in neighboring Iraq, was responsible for the Katyusha attack.
Washington blamed that militia for a strike in December that killed a U.S. contractor and triggered a round of violence that led U.S. President Donald Trump to order the killing of a top Iranian general, Qasem Soleimani, in a drone strike in Baghdad the following month.
In retaliation, an Iranian ballistic missile strike on an Iraqi air base left some 110 U.S. troops suffering from traumatic brain injuries.