Unidentified planes struck targets in Syria near the border with Iraq on Friday, killing eight Iran-backed Iraqi militiamen, Syrian activists and two Iraqi officials said, an attack that comes amid soaring tensions between the US and Iran.

The Syrian Observatory for Human Rights said the planes targeted positions belonging to pro-Iran militias in the Boukamal area, near the border with Iraq.

The Britain-based organisation, which documents the war in Syria through a network of activists on the ground, said the planes struck, among other targets, weapons depots and vehicles belonging to the militias.

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An Iraqi security official and another official, from the Iran-backed Iraqi militias known as the Popular Mobilisation Forces, said warplanes targeted two vehicles carrying missiles on the Syrian side of the border.

The strike was most likely carried out by Israeli warplanes, they said but offered no evidence.

The Iraqi officials identified the eight casualties as Iraqi militia fighters while the Observatory only said the eight were not Syrians, without giving their nationality.

The death toll could rise further, officials said, as there were also wounded militiamen, some reportedly in serious conditions.

Another Iraqi official said those targeted belong to the Imam Ali Brigades, an Iran-backed faction within the PMF.

The three Iraqi officials spoke on condition of anonymity because they were not authorised to make statements to the media.

Israel has repeatedly struck Iran-linked targets in Syria in recent years and has warned against any permanent Iranian presence on the frontier.

Deir Ezzor 24, an activist collective that reports on news in the border area, said that the planes strikes trucks carrying weapons and depots for ballistic missiles in the area.

Omar Abu Laila, a Europe-based activist from Deir el-Zour who runs the group, said the attack triggered a huge explosion heard in the Syrian-Iraqi border.

The sound and picture, another activist collective in Syria's eastern Deir el-Zour area, said "unidentified planes" struck militia targets in Boukamal. There was no immediate comment from Syria authorities.

The US carried out military strikes in the area on December 29, killing 25 members of an Iran-backed Iraqi militia in retaliation to a rocket attack on a military base in Iraq that killed a US contractor. The US blamed that attack on a Iranian-backed Iraqi militia.

The reported airstrikes came days after a US drone strike killed Iran's most powerful general after he landed at Baghdad airport, drawing angry calls for revenge and escalating tensions to the brink of an all-out war between the two sides.

Iran responded by firing a barrage of missiles at military bases in Iraq that host US troops.

Since then, both sides have signalled they were stepping back from further escalation but tensions remain high and the region on edge.

Amid the soaring tensions, Russian President Vladimir Putin made a rare visit to Syria this week for talks with President Bashar Assad in Damascus.

Russia has been a key ally for Assad, offering crucial military and political backing throughout the country's civil war.

The area struck on Friday is key to a land corridor for Tehran that links Iran across Iraq and Syria through Lebanon.

The Observatory report on Friday claimed that Putin had informed Assad during the visit of a US intention to close the land corridor for good.