U.S. officials say they see Iran’s hand in the increasingly brutal crackdown on opposition strongholds in Syria, including evidence of Iranian military and intelligence support for government troops accused of mass executions and other atrocities in the past week.

Three U.S. officials with access to intelligence reports from the region described a spike in Iran­ian-supplied arms and other aid for Syrian leader Bashar al-Assad at a time when the regime is mounting an unprecedented offensive to crush resistance in the key city of Homs.