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.
Syrian Air Defenses thwarted an Israeli attack on Tal al-Hara in southern Syria and shot down a number of them, Syrian state news agency SANA said early on Wednesday.
After targeting the outskirts of the capital Damascus, Israel "resumed its aggression and launched several missiles against the east of Quneitra province killing three people and wounding seven other soldiers", the official SANA news agency reported.
New Zealand reported two new coronavirus cases on Saturday -- one in managed isolation and the other detected during contact tracing, taking the overall tally to 1,475, according to the Ministry of Health.
Togolese Prime Minister Komi Selom Klassou and his government tendered their resignation on Friday, the presidency said in a statement, congratulating the Cabinet for its work in office.
South Korea said on Saturday it will request North Korea to further investigate the killing of a South Korean government official who was shot by North Korean troops after being found adrift near the rivals’ disputed sea boundary while apparently trying to defect.
US President Donald Trump said on Friday that Americans might not know the winner of the November presidential election for months due to disputes over mail ballots, building on his criticism of a method that could be used by half of US voters this year.