Beirut - Drones attacked a convoy of trucks in eastern Syria Sunday night shortly after it crossed into the country from Iraq, Syrian opposition activists and a pro-government radio station said. There was no immediate word on casualties.

The strike comes amid heightening tension between Iran and its rivals in the region.

It was not immediately clear who was behind the attack on the convoy in the Syrian border region of Boukamal, which is a stronghold of Iran-backed militias.

The Britain-based Syrian Observatory for Human Rights, an opposition war monitor, said the drones appear to have been from the U.S.-led coalition, adding that they targeted six refrigerated trucks. The group said there were casualties and ambulances rushed to the area.

Another activist said the strike hit a convoy of trucks of Iran-backed militiamen. Omar Abu Layla, a Europe-based activist from Deir el-Zour who runs a group that monitors developments, tweeted that there was no immediate word on casualties.

The pro-government Sham FM radio station also reported that six refrigerated trucks were hit.

In Baghdad, an official with an Iran-backed militia confirmed there was a strike saying it only targeted one truck. He gave no word on casualties.

The attack in eastern Syria came hours after bomb-carrying drones targeted an Iranian defense factory in the central city of Isfahan causing some damage at the plant.

Last month, Israel's military chief of staff strongly suggested that Israel was behind a strike on a truck convoy in Syria in November, giving a rare glimpse of Israel's shadow war against Iran and its proxies across the region.

Lt. Gen. Aviv Kochavi, who finished his military service earlier this month, said Israeli military and intelligence capabilities made it possible to strike specific targets that pose a threat.

Israeli leaders have in the past acknowledged striking hundreds of targets in Syria and elsewhere in what it says is a campaign to thwart Iranian attempts to smuggle weapons to proxies like Lebanon's Hezbollah militant group or to destroy weapons caches.

The November strike hit tanker trucks carrying fuel and other trucks carrying weapons for the militias in Syria's eastern province of Deir el-Zour, the Observatory reported at the time. It said at least 14 people, most of them militiamen, were killed in the strike.

The strike, along the border with Iraq, targeted Iran-backed militiamen, Syrian opposition activists said at the time. Some of those killed in the attack were Iranian nationals, according to two paramilitary officers in Iraq.

At the time, Israel declined to comment on the strike.

Iran is a main backer of Syrian President Bashar al-Assad and has sent thousands of Iran-backed fighters to help Syrian troops during the country's 11-year civil war. Both Iran and Assad's government are also allied with Hezbollah, which has fought alongside Assad's forces in the war.

Israel considers Iran to be its chief enemy and has warned against what it views as its hostile activities in the region.