Russian military police last week reportedly carried out a raid against Iranian-backed militiamen stationed at Syria's Aleppo international airport, local media reported.
In the aftermath, several Iranian militia leaders were arrested in what was seen as the latest episode of tensions between Iranian and Russian forces in Syria.
Since the beginning of Syria's civil war in 2011, Russia and Iran have built a strong military presence in the country in support of forces loyal to Syrian President Bashar al-Assad's regime.
Iran has since deployed thousands of its Islamic Revolutionary Guard Corps (IRGC) and allied Shiite militias to Syria, while Russia officially entered the Syrian conflict in September 2015 to help Assad's regime.
But as the war is waning, with Syrian regime forces reclaiming most of the territory once controlled by rebel forces, Russia and Iran seem to be vying for influence in the war-torn country.
'Slice of the pie'
Analysts say the protracted war in Syria has created a slight fissure between the two allies.
"There are definite tensions that exist between Russia and Iran within Syria," said Phillip Smyth, a researcher at the Washington Institute for Near East Policy who closely follows Iranian-backed militias in Syria.
"You see things like this [raid in Aleppo] that occur in flashpoint zones because there's criminal activity going on. Each country's proxy wants a cut of that," he told VOA.
Similar incidents have been taking place throughout the country in the past two years.
Recently, two divisions of the Syrian military were engaged in deadly clashes in different parts of the country, local reports said.
This power struggle is the result of differences among Syrian military leaders who are either loyal to Russia or Iran, observers believe.
"I do believe that it comes down to who controls what, what slice of the pie they all have. But I don't necessarily believe that this is going to lead to some major conflagration between Iranian and Russian forces there," analyst Smyth said.
The strategic partnership between Russia and Iran in Syria goes beyond such disagreements, especially since Russia is still dependent on Iranian forces to hold territory and to provide manpower for Syrian regime troops, some experts say.
"I never believe that Russia would separate from Iran," said Anna Borshchevskaya, a research fellow at the European Foundation for Democracy who focuses on Russia's policy in the Middle East.
"The disagreements they're having is that they're trying to carve out spheres of influence in Syria, which is something that Russia understands very well," she told VOA in a phone interview. "Their relationship is a complex one, for sure. But what holds them together is their anti-Americanism and a desire to reduce American influence in the region."
Borshchevskaya added that "on the tactical level, [Russia and Iran] are going to have differences sometimes. But they agree on the big picture."
The U.S. has been involved in the war against Islamic State militants since 2014, when the terror group announced its so-called caliphate in Syria and Iraq.
U.S.-backed Syrian Democratic Forces (SDF), who declared victory over IS in March, now control more than a third of Syria's territory.
The United States has about 2,000 troops in areas under the control of the Kurdish-led SDF. But the U.S. administration has said it will keep only about 400 soldiers in those areas after the war against IS is over.
Russia and Iran have constantly opposed the U.S. military presence in Syria.
Some analysts believe that, unlike when they became involved in Syria's war, Russian and Iranian forces now control larger territories and both countries are searching for economic opportunities in the country.
"Now there are more points of friction between the two countries than ever before," said Jowan Hemo, a Syrian economist who follows the economic patterns of the war.
"So naturally, you would see them compete to win contracts with the Syrian regime, including the energy and power sectors and other types of investments," he told VOA.
In 2018, Russia was awarded exclusive rights to produce Syria's oil and gas. Russia has also signed a contract to use the Syrian port of Tartus for 49 years, while Iran won a bid to partially use the port of Latakia.
Both countries want to economically monopolize Syria for the long term, because they each have given sizable loans to the Syrian regime throughout the war, economist Hemo said.
"I believe this type of competition will continue in Syria, but eventually Russia's economic dominance will prevail," he added.