Members of the Syrian opposition said Sunday as many as 100 Russian advisers have left the country, sending a worrying signal to President Bashar Assad: One of his top financial and military backers in the 4-year-old civil war may be backing out, leaving the teetering Syrian Army to fight almost entirely on its own. Regime forces are facing the Islamic State group and the increasingly powerful umbrella group known as Jaysh al-Fatah, which scored a major victory last week when it took over the northern province of Idlib, routing Assad’s soldiers.

The Russians left Syria -- on an airplane that took off from the regime-held city of Latakia in the southwestern part of the country -- on orders from their government, Saudi-backed newspaper Asharq al-Awsat reported Sunday, citing sources with the Syrian opposition. That happened, the sources said, as Russia started meetings with gulf countries, seeking to boost economic ties to offset the impact of the sanctions the U.S. and European Union imposed on Russia for its intervention in the war in Ukraine.

The governments of Sunni Muslim states in the gulf, led by Saudi Arabia, staunchly oppose Assad’s regime, which is dominated by Alawite Muslims, an offshoot of Shiite Islam.

Syrian opposition officials told the newspaper those who left included experts who were stationed in Damascus and assisted the Syrian regime on battlefield strategy, as well as advisers from Iran and Hezbollah, the Iranian proxy in Lebanon that has been helping prop up the regime. Still, experts told the Jerusalem Post, although “Russia does not trust Assad 100 percent,” it is not likely to pull all military support.

News of the departures comes just one week after U.S. Secretary of State John Kerry visited Russia where he hinted at a press briefing the U.S. and Russia were looking to begin collaborating on Syria after four years of stark differences.

Rebels fighting the regime north of Aleppo, Syria’s biggest city, told the International Business Times the news of the departures had yet to reach their ranks. But those rebels, who previously were backed with weapons by the U.S. and are now a part of the Islamist-dominated Jaysh al-Fatah, said they welcomed any word of a weakened Assad and the chance to gain more ground near the capital.

In fact, these rebels have for months considered joining a U.S.-run training program in Turkey, but declined because under the terms of the program they would not be allowed to fight Assad, only the Islamic State group, also known as ISIS.

The loss of support for Assad means that the rebels now have less need for U.S. training and weapons to advance against an increasingly ineffective Syrian military.

The announcement of the removal of Russian military advisers came at the same time Saudi Foreign Minister Adel al-Jubeir, in a briefing for reporters during a visit to Cairo, said his country was actively trying to convince Russia to stop supporting Assad altogether. He also took a stab at Iran, Assad’s other main supporter and the main supporter of Hezbollah.

"We reject the actions of Iran ... and Iran's support for terrorism," al-Jubeir said. "[Iran] is the only state that interferes in the affairs of the region, whether in Lebanon or Syria or Iraq or Yemen.”

Iran and Russia, Syria’s traditional weapons supplier, have supported Assad since 2011 while the U.S. and its allies have tried to prop up moderate rebels. The removal of Russian and Iranian advisers points to the possibility the battle is turning against the regime in the long term, a sudden reversal for Assad, who has been able to hold on through four years of war thanks to his military superiority -- a superiority that was largely a product of Russian and Iranian help.

Assad is the only player in the Syrian war with an air force, but its planes, weapons, spare parts and training are all provided by Russia. Without Moscow’s support, the regime will progressively lose its key advantage in the sky. And on the ground, it has relied increasingly on men, supplies and money from Hezbollah, a U.S.-designated terrorist organization that has been de facto a part of the regime’s militias.

The removal of their influence in Syria would be a major win for the U.S. and its Western allies, which in the past four years have tried to pass U.N. Security Council resolutions that would have, among other things, referred the Syrian government to the International Criminal Court for prosecution stemming from crimes against humanity. Russia and China have both vetoed those resolutions.

Assad’s removal from the equation in Syria would also have consequences for the U.S.-led fight against ISIS.

The U.S. is currently training and arming hundreds of Syrian opposition fighters at a base in Jordan, and has been trying to ramp up a similar program in Turkey. But according to fighters in the ranks of Jaysh al-Fatah, the rebel group in Idlib, the Turkish training program has yet to begin because of disagreements between rebels and the U.S. about strategy. The U.S. wants to focus on defeating ISIS while the rebels want the plan to include taking out Assad.

If the weakening of the regime is not accompanied by a buildup of rebel forces in Syria, the withdrawal of Russian and Iranian military officials could backfire for the anti-ISIS effort. The Assad regime has been on the same side as the West in the fight against ISIS, and Russian arms have helped defeat the Sunni militant group in parts of the country, especially near Damascus.