WASHINGTON — Syrian forces loyal to President Bashar Assad have fired Scud missiles at rebel fighters in recent days, Obama administration officials said Wednesday.

The move represents a significant escalation in the fighting, which has already killed more than 40,000 civilians in a nearly two-year-old conflict that has threatened to destabilize the Middle East, and suggests increased desperation on the part of the Assad government. A fresh wave of mayhem struck the Syrian capital Damascus on Wednesday, reports from the region said, including a deadly triple bombing outside the Interior Ministry.