A Russian force deployment on the Syria-Lebanon border this week in a Hezbollah stronghold sparked protests by the Lebanese militant group, prompting the force to withdraw from its positions only a day later in a rare sign of tension between the allies.
The Russian move was not expected as Moscow's military police have been deploying in areas controlled by Syrian government forces and close to insurgent positions. The outskirts of the Syrian town of Qusair where the Russian troops set up three observation positions on Monday have been held by Hezbollah and Syrian troops since 2013, when they drove rebels from the area.
The Russian deployment and subsequent withdrawal shows that as rebels are being defeated in different parts of Syria, frictions could rise between Syrian President Bashar Al Assad's main foreign backers — Russia and Iran — and the militias Tehran backs throughout Syria.
"They came and deployed without co-ordination," said an official with the so-called "Axis of Resistance" led by Iran, which includes Iran, Syria, Hezbollah and other groups fighting alongside the Assad forces.
"It's better if they don't come back. There is no work for them there. There is no Daesh or any other terrorist organisation," the official said, referring to ISIS and other insurgents that the Syrian government and its allies call terrorist organisations. "What do they want to observe?" he asked.
Asked if there is tension between Hezbollah and Russian troops, the official refused to comment, speaking to the Associated Press by telephone from Syria on condition of anonymity because he was not authorised to brief reporters. He said that after the Russian troops left, Syrian forces belonging to the army's 11th Division replaced them.
In 2013, Hezbollah openly joined the Syrian civil war along with Mr Assad's forces capturing the then rebel stronghold of Qusair in June that year after losing dozens of its battle-hardened fighters.
The Russian deployment outside Qusair came after Israeli warplanes struck the nearby Dabaa airbase on May 24, according to Syrian activists who said Hezbollah arms depots were hit. There was no word on casualties.
The Israeli military is believed to be behind dozens of air strikes in recent years against Hezbollah, Iran, and Syrian military positions. The US and Israeli governments have viewed Iran's role in Syria as a threat to Israel and have threatened action.
Although there have been no reports of frictions between Russian and Iranian or Iran-backed fighters in Syria, calls for Tehran to end its military presence in Syria have been on the rise in recent weeks.
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The Iranian militias fighting in Syria
Netanyahu warns Syria: expel Iran or face military consequences
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At a meeting with Mr Assad, who visited the Russian city of Sochi last month, Russian President Vladimir Putin noted that a political settlement in Syria should encourage foreign countries to withdraw their troops.
Mr Putin's envoy for Syria, Alexander Lavrentyev, later commented that the Russian leader's statement was aimed at the United States and Turkey, along with Iran and Hezbollah. It marked a rare instance in which Moscow suggested Iran should not maintain a permanent military presence in the country.
US Secretary of State Mike Pompeo issued a list of demands last month for a new nuclear deal with Iran, including the pullout of its forces from Syria. Israel has also warned it will not accept a permanent Iranian military presence in Syria.
Syrian Deputy Foreign Minister Faisal Al Mokdad however has said on Russia's Sputnik news agency that "this topic is not even on the agenda of discussion, since it concerns the sovereignty of Syria".
A top Iranian security official said that Tehran will maintain an advisory role in Syria and continue to support "resistance groups". The secretary of Iran's Supreme National Security Council Ali Shamkhani meanwhile told Al Jazeera TV that as long as Syria faces a "terrorist" threat and Damascus requests its presence, "we will stay in Syria".
And for his part, Hezbollah leader Hassan Nasrallah said in a speech on Friday that "if the whole world tried to impose on us a withdrawal from Syria they will not be able to make us leave", adding that his group would only leave at the request of the Syrian government.
The tensions come amid escalation in the country's southwest near the border with Israel, where in early May Iran struck Israeli positions in the Golan Heights in retaliation for repeated air strikes in Syria.
Read more: Israel suspected of carrying out new air strikes in Syria
On May 10, Israel unleashed a heavy bombardment against what it said were Iranian military installations in Syria. It said it was retaliation for an Iranian rocket barrage on its positions in the Golan. It was the most serious military confrontation between the two bitter enemies to date.
Israel has been mostly using Lebanon's airspace to strike targets inside Syria in an apparent move to avoid any conflict with Russia's warplanes that fly over Syria. Russia has a major airbase near Syria's coast from where warplanes have been taking off to strike at insurgents throughout Syria.
"There is an increasing evidence that shows that Russia has turned a blind eye to Israel's air strike in Syria against Iran's military presence," said Fawaz Gerges, professor of Middle Eastern politics at the London School of Economics. "This is a direct message that Russia does not want Iran to have a hegemonic position in Syria."
Russia and Iran have been the main backers of Mr Assad but Moscow also has close relations with Israel whose Prime Minister Benjamin Netanyahu visited Russia several times over the past two years. On one trip last month, he stood close to Mr Putin while attending a massive parade for Russian troops marking victory in the Second World War.
Russia has been reportedly mediating for Iranian troops and Hezbollah fighters to withdraw from areas close to the Israeli border where Syrian troops are expected to launch an offensive against rebels.
"What happens after is not Russia's problem: Iran will fight Israel for centuries. Netanyahu won't be satisfied with Iran's exit from south-west Syria, he needs an Iran-free Syria, which is impossible now or ever," said Maxim Suchkov, who edits Russia-Middle East coverage at online news website Al Monitor and sits on the Russian International Affairs Council. "Neither Russia, nor anyone can ensure that."
Since September 2015, Mr Assad's forces have been making strong gains on the ground against insurgents thanks to Russian air cover and ground forces mostly made up of Iran-backed fighters from Lebanon, Iraq and Afghanistan. Mr Assad now controls more than half of Syria's territories including the largest four cities.
Russian troops do not appear to be leaving Syria, home to their only naval base outside the former Soviet Union, any time soon. The Russian parliament voted in December to extend Russia's lease of the naval base in the Syrian city of Tartous for 49 years, following Vladimir Putin's announcement of a partial pullout of Russian troops from the war-torn country.
"In the past three years, Russian and Iranian influence converged in Syria. They wanted to rescue the Assad regime," Mr Gerges said. "Now that we are witnessing the beginning of the end of the military phase, we are witnessing divergence of interest between Russia and Iran."