Syrian officials described the incident as a violation of international law and a "terrible crime."
The Associated Press quoted an unnamed American military official who confirmed the operation, saying, "We are taking matters into our own hands."
As is typical in these cases, no US official would comment on the record on the alleged attack, including US Special Operations Command and Marine officials in Anbar Province in Iraq. Bryan Whitman, a Pentagon spokesman, refused to comment on the incident Monday.
The US has reportedly begun cross-border attacks in Pakistan in recent months in pursuit of militants seeking haven in the border tribal areas. The New York Times reported last month that President Bush signed an accord in July allowing US Special Operations forces to conduct ground assaults inside Pakistan's borders. No American officials confirm the existence of such an accord.