The Observatory did not identify the responsible, but its head Abdul Rahman told the media that Israel was likely responsible.
At least 12 pro-Iranian fighters died in strikes by unidentified aircraft on eastern Syria late Saturday evening, a war monitor said.
"Eight air strikes before midnight on Saturday night targeted a base of pro-Iranian forces in rural eastern Deir Ezzor (province), killing 12 Iraqi and Afghan fighters and destroying equipment and ammunition," the Syrian Observatory for Human Rights said.
The Observatory did not identify the aircraft responsible, but its head Rami Abdul Rahman told the media that Israel was likely responsible.
The Jewish state has carried out hundreds of strikes targeting regime and Iranian-backed forces, notably in Deir Ezzor.
The Israeli military rarely claims responsibility for such attacks but has vowed to prevent Iran gaining a foothold in the war-torn country or delivering advanced weaponry to Lebanese armed group Hezbollah.
Iranian and Iraqi armed groups backing the regime of Bashar Al Assad have deployed across swathes of Deir Ezzor, a large desert province bordering on Iraq.
The Observatory said the latest strikes came after Afghan forces brought in reinforcements from near the Iraqi border to a large Iranian base near the town of Al-Mayadin on the Euphrates river.
Two waves of similar strikes in May killed 12 pro-Iranian fighters, according to the Observatory.
Syria's complex, almost decade-long war has killed over 380,000 people, devastated the country's infrastructure and forced millions of people to flee their homes.
A high-ranking Iranian official on Sunday denied Iranian positions had been hit by Israeli air strikes near the Syrian capital overnight.
Since the beginning of the war in Syria in 2011, Israel has conducted hundreds of strikes in Syria, most of them against what it says are Iranian and Hezbollah targets.
The Israeli air force carried out more than 18 strikes against multiple targets in an area stretching from the eastern town of Deir Ezzor to the Boukamal desert at the Syrian-Iraqi border, according to the Britain-based war monitor.
Eleven residential buildings and 39 houses were damaged in the explosions which occurred in the early hours.
The UAE has sent three planes carrying a 1,000 sqm field hospital, which includes 75 beds and two operating rooms equipped with the latest medical supplies and devices, to offer relief to earthquake victims in Afghanistan.
Residents in the UAE felt strong tremors and left their residences. A family told Gulf Today that as soon as they realise that their chandellier was moving without any reason. they knew it was earthquake.
On the other hand, the Gaia space probe unveiled its latest discoveries in its quest to map the Milky Way in unprecedented details, surveying nearly two million stars and revealing mysterious "starquakes" which sweep across the fiery giants like vast tsunamis.