Israeli air strikes on east Syria killed 57 regime forces and allied Iran-backed fighters, in the deadliest such strikes since the start of the conflict, a war monitoring group said Wednesday.

The overnight raids against arms depots and military positions killed at least 14 Syrian regime forces, 16 Iraqi militia fighters and 11 Afghan members of the pro-Iran Fatimid Brigade, said the Syrian Observatory for Human Rights.

The nationality of the remaining 16 who lost their lives in the 18 strikes was not immediately clear.

"This is the largest death toll from Israeli raids in Syria," Observatory head Rami Abdul Rahman told AFP.

The official Syrian news agency SANA said "the Israeli enemy carried out an aerial assault on the town of Deir Ezzor and the Albu Kamal region", only adding that "the results of the aggression are currently being verified".

Contacted by AFP, an Israeli army spokesperson declined to comment.

Days before the strikes, the Fatimid Brigade transported a consignment of Iranian-manufactured weapons to eastern Syria from neighbouring Iraq, said the Observatory, which is based in the United Kingdom.

They were stored in the region targeted overnight, it added.

In June 2018, strikes on the same region killed at least 55 pro-government fighters, including Iraqis as well as Syrians, according to the monitoring group, which relies on a network of sources on the ground.

The latest raids came hours after separate strikes near the Iraqi border killed at least 12 Iran-backed militia fighters on Tuesday.

The Observatory said it could not identify the warplanes responsible for the earlier strikes.

- 'Not sitting and waiting' -

It is less than a week since the last wave of Israeli strikes in Syria.

On Thursday, Israel targeted positions in the south and in the southern outskirts of Damascus, killing three pro-Iran fighters.

Israel routinely carries out raids in Syria, mostly against targets linked to Iran in what it says is a bid to prevent its arch foe from consolidating a foothold on its northern border.

Israeli strikes in Syria / © AFP

Observers have warned that Donald Trump and Israel could up the ante against Iran and its regional allies in the final days of the US president's tenure.

Trump's administration, which is to make way for Joe Biden's on January 20, gave unprecedented US support to the government of Israeli Prime Minister Benjamin Netanyahu.

"In the dying days of the Trump administration, Netanyahu is trying to do as much damage as possible to the IRGC effort in Syria before Biden takes office" said Nicholas Heras, of the Institute of the Study of War, referring to Iran's Islamic Revolutionary Guard Corps.

Israeli Defence Minister Benny Gantz on Tuesday said: "We remain vigilant on all our frontiers."

"We have taken action and will continue to take action against anyone who tries to challenge us, from near or far. We're not sitting and waiting," he added.

Israel hit around 50 targets in Syria in 2020, according to an annual report released by the Jewish state's military.

It has carried out hundreds of air and missile strikes on Syria since civil war broke out in 2011, targeting Iranian and Lebanese Hezbollah forces as well as Syrian government troops.

Israel rarely acknowledges individual strikes but has done so when responding to what it describes as aggression inside Israeli territory.

The war in Syria has killed more than 387,000 people and displaced millions more since it erupted after the brutal repression of anti-government protests.