Israeli official: Iran's military mastermind went to Russia to talk to Putin about saving Assad
In August, two unnamed Western intelligence sources told Fox News that Soleimani had violated a travel ban and sanctions to meet with Russian Defense Minister Sergei Shoigu and President Vladimir Putin in Moscow.
The official added that cooperation between Russia and Iran has resulted in a significant military buildup in the Latakia province of western Syria.
"Russia ... has teamed up with Iran in an unprecedented attempt to protect the embattled regime of Bashar Assad from falling to rebel groups including the Islamic State," YNet reported.
Soleimani - a major general in the Iranian Revolutionary Guards Corps (IRGC) and commander of the powerful Quds force - has reportedly sent hundreds of ground soldiers into Syria over the past few days.
Meanwhile, Russian drones and fighter planes have been surveilling non-ISIS rebels in the country's north.
Russian armored-personnel carriers with Russian-speaking troops have also apparently been involved in fighting, and Russia has set up an air-traffic-control tower and brought housing units for up to 1,000 personnel to Latakia in the country's west. Two tank-landing ships and additional aircraft have arrived, Reuters reported.
"It's hard to forecast whether Russia's presence will decide the fate of Syria, but it will lengthen the fighting and bloodletting for at least another year because ISIS won't give up," the Israeli source told YNet.
Under Soleimani's purview, has long since taken over Assad's fight in crucial parts of Syria.
In May, Soleimani traveled to Syria to "organize the entry of Iranian officials to supervise and aid" Iranian proxy forces in coastal Syria, according to Now Lebanon.
One month later, a Free Syrian Army (FSA) commander told the London-based Al-Quds al-Arabi that "the regime has handed over the operations room to Iranian officers and leadership."
"Syrian officers, among them Alawites, have become secondary members, whose tasks can sometimes be reduced to handing out tea and coffee," he added.
Now, it appears that Iran is upping the ante in coordination with Russia.
"Assad has lost significant territory over the past months; Putin is not about to tolerate his ouster," geopolitical expert Ian Bremmer, the president of Eurasia Group, told Business Insider in an email.
Iran is not about to tolerate it, either - Iran is making "an effective play for regional hegemony," Michael Gerson of the Washington Post has written, and needs Assad to remain in power to maintain its bridge to Hezbollah in Lebanon.
Russia, too, has its own interests to look after: control over its naval base at Tartus is at stake, and along with it Putin's ability to project power into the Mediterranean.
As a result, Russia's incursion into Syria has less to do with fighting ISIS and more to do with countering Western actions that have bolstered rebel forces in the north and threaten to weaken Assad further.
"If the West succeeds in turning the tide of the war while Assad is vulnerable, the political outcomes in Syria are more likely to be dictated by the US," Bremmer said. "Which means Putin needs to bolster Assad now."
Michael B. Kelley contributed to this report.