A US official has blamed Israel for a deadly strike along Syria's eastern border with Iraq, raising many questions over the target and victims of the raid. This is what we know:
Where was it?
Late Sunday, an air strike hit the border town of Al-Hari in eastern Syria near the frontier with Iraq, state media and a Britain-based monitor said.
The raid in the eastern province of Deir Ezzor killed 55 fighters including Syrians and Iraqis, the Syrian Observatory for Human Rights monitoring group said.
Deir Ezzor is the backdrop of two separate offensives against the Islamic State jihadist group, and a de-confliction line exists to keep them from running into each other.
Al-Hari lies on the western banks of the Euphrates River, where regime forces have been battling the jihadists with the help of Russian aircraft and foreign fighters.
On the eastern side of the waterway, a Kurdish-led alliance supported by the US-led coalition has been pressing a campaign against IS.
The jihadists declared a cross-border "caliphate" in Syria and Iraq in 2014, but have since seen their presence largely confined to a few holdouts in Deir Ezzor.
Al-Hari also sits along a key route linking the Syrian-Iraqi border, and Iran beyond that, all the way west to the frontier with Lebanon.
That road could be crucial for the transport of weapons, equipment, and high-value personnel across territory held by Syria's regime and its regional allies, analysts say.
Who was targeted?
Iraq's powerful Hashed al-Shaabi paramilitary force, which has fought IS in Iraq, said 22 of its fighters lost their lives in the strike.
The bodies of at least three fighters of the Hezbollah Brigades, part of the Hashed, have been repatriated to southern Iraq, an AFP correspondent said.
The group is among several pro-Iran groups present inside Syria including along the border with Iraq, sources within the Hashed said.
These groups have fought alongside Damascus regime fighters since the start of Syria's civil war in 2011, before joining the Hashed to expel IS from Iraq in 2014.
Observatory head Rami Abdel Rahman said Sunday's target was used as "the headquarters of the Iranian forces responsible for the operations in the eastern countryside of Deir Ezzor".
The deadly raid hit "as a convoy of the Hezbollah Brigades stopped nearby," he said.
Who did it?
Both Damascus and the Iran backed-Hashed have blamed the US-led coalition fighting IS.
But the coalition and the Pentagon denied any involvement.
Pentagon spokesman Eric Pahon said: "The strike was not conducted by the US or the coalition."
Instead, a US official pointed the finger at Israel.
"We have reasons to believe that it was an Israeli strike," the official told AFP on condition of anonymity on Monday.
Last month, Israel launched a large-scale attack on what it said were Iranian targets in Syria, raising fears of a major confrontation.
Those strikes followed a barrage of rockets that Israel said was fired towards the occupied Golan by Iranian fighters from Syria.
Even before that, Israel had been blamed for a series of strikes inside Syria that killed Iranians, though it has not acknowledged them.
If Sunday's strike was carried out by Israel, it would mark a significant escalation against its Iranian foe, said Phillip Smyth, an analyst at the Washington Institute for Near East Policy.
"If it were the Israelis, then they are sending a signal that we can hit you all the way out there and we can hit a pretty nasty organisation, the Hezbollah Brigades," Smyth told AFP.
How did all sides react?
Israel declined to comment Tuesday.
But just hours before the attack, Israel's premier warned his country would act against Iranian presence in neighbouring Syria.
"Iran needs to withdraw from all of Syria," Prime Minister Benjamin Netanyahu said.
"We will take action -- and are already taking action -- against efforts to establish a military presence by Iran and its proxies in Syria both close to the border and deep inside Syria," the prime minister said.
"We will act against these efforts anywhere in Syria."
The Hezbollah Brigades, which lost fighters in the raid, on Tuesday denounced "an aerial strike by the American-Zionist demon" who wants to maintain "IS and its affiliates".
"This heinous crime will open the way to a new confrontation with the Zionist entity and the American project, and the Hezbollah Brigades will not hesitate to go towards this confrontation."
© 2018 AFP