Iran has started to withdraw its ground troops from Syria, U.S. and other Western officials told Bloomberg. 

According to the report, U.S. officials have seen a significant reduction in Iranian presence in Syrian combat zones.  One estimate puts the number of Iranian Revolutionary Guard troops currently fighting in the country at 700, a drop of over 1,000 from previous estimates. 

According to Bloomberg, U.S. officials believe the pullout is due to the high casualty rates suffered by the Iranians, including the reported injuries suffered by Qassem Suleimani, the Guard's elite Quds Force commander. 

Iran sent about 2,000 Revolutionary Guards to Syria in September as part of a plan with Russia for a ground operation in northern Syria to back up Russian airstrikes.  Israeli observers have noted Iran’s and Hezbollah’s unwillingness to suffer high casualties.  Hezbollah's losses since the summer of 2012, when it began direct involvement in the Syrian civil war, total more than 1,300 killed and 5,000 wounded.  This is high considering that probably no more than 30,000 men are serving, including reserves. 

Despite the wider Russian involvement since September, no major ground victories against the rebels have been achieved.  The Assad regime currently controls about 15 to 20 percent of Syrian territory.