Ambassador Robert S. Ford, addressing a forum in Washington, also warned that Islamist terrorists are getting their hands on weapons supplied to the rebels from other countries. He insisted that the United States is not directly sending arms to the rebels.
“This is a huge, huge humanitarian crisis,” Mr. Ford told the Middle East Institute, speaking as part of a panel discussion last week.
More than 450,000 Syrians have fled the country and become refugees in Jordan, Iraq, Lebanon and Turkey — creating massive economic and social burdens on those nations, he said. An additional 1 million people have fled their homes but remain in Syria.
“The internally displaced [Syrians] have skyrocketed,” Mr. Ford added.
The ambassador, who left Syria in February when the United States closed its embassy in Damascus, denounced Mr. Assad for ordering his forces to attack civilian areas, especially hospitals, in rebel-held regions.
“These are genuine war crimes,” he said.
On Sunday, Syrian government warplanes and artillery pounded parts of Damascus and the capital’s suburbs where rebels have advanced. In the restive central city of Homs, where the rebellion broke out nearly 22 months ago, a car bomb exploded, killing at least 15 people and injuring 24 others, according to Syria’s state-run SANA news service.
An increase in car-bomb attacks is seen as evidence of a growing presence of Islamist terrorists linked to al Qaeda.
He identified the terrorist group as Jabhat al-Nusra, warning that it “poses a real danger” to reaching a political solution to the conflict, which activists say has claimed more than 40,000 lives.
Mr. Ford added that the United States is “strongly, strongly, strongly” supportive of the new Syrian opposition coalition, formed last month in Doha, Qatar.
Britain, France, Turkey and several Arab nations have recognized the National Coalition of Syrian Revolutionary and Opposition Forces as the legitimate representative of the Syrian people.
Mr. Ford noted that the United States, which has donated $197 million in aid, is the largest single supplier of relief aid for the Syrian humanitarian crisis.