"So far we have no evidence to substantiate the reports that chemical weapons were used yesterday," Robert Ford told the House Foreign Affairs Committee. He later added that the administration was extremely concerned and trying to verify reports that such weapons were used in the Aleppo province and the Damascus suburbs.
The administration on Tuesday disputed Assad's claim, and a U.S. official said there was no evidence that either Assad's forces or the opposition had used chemical weapons in an attack.
President Barack Obama has declared the use, deployment or transfer of the weapons to be his "red line" for possible military intervention in the Arab country. Ford said Wednesday that an increasingly besieged Assad regime might be tempted to use its stockpile of chemical weapons, the largest in the region.
Hours before Ford's testimony, the chairman of the House Intelligence Committee, Rep. Mike Rogers, said he feared a "stain on our national credibility" if the United States determines Assad is using chemical weapons to remain in power but fails to intervene.
The Michigan Republican said there was a "high probability" that a chemical agent was deployed Tuesday in northern Syria.
Rogers told "CBS This Morning" the U.S. knows "there has been some forensic evidence that at least small quantities" of chemical weapons may have been used. Rogers also said the United States had "lost the faith" of the opposition forces, adding, "This is the time to act. Don't wait until we have 5,000 dead."
In his testimony, Ford described an untenable situation as the Syrian civil war grinds on into its third year. The United Nations has estimated 70,000 have been killed, more than 1 million refugees have fled to neighboring countries and 2.5 million have been displaced internally.
The Syrian people "face a new level of ruthlessness from the Assad regime, which is raining Scud missiles down on residential neighborhoods, destroying hospitals and schools, and sending its thugs rampaging through the streets to terrorize their fellow citizens. The carnage is appalling," Ford said.
He insisted that the ideal outcome is a "negotiated political transition" to the crisis without Assad.
Ford said the military balance is turning against the Assad regime, which has lost some critical strategic locations such as the borders with Turkey and Iraq. The ambassador also said there has been heavy fighting in Damascus "right up close to where the president lives."
Ford was pressed repeatedly about what military action the United States might take but declined to speculate at the public hearing. Lawmakers uneasy with military involvement - or even the prospect of arming the opposition - reflected a war-weariness after more than a decade of conflicts in Iraq and Afghanistan.
Ford underscored that Obama has said there will be consequences if chemical weapons were used and said he was certain that there have been discussions within the administration about possible steps.
"I do want to underline how seriously we take the reports," he said.
Republican Rep. Scott Perry of Pennsylvania, who noted the 10th anniversary of the invasion of Iraq this week, repeatedly tried to get Ford to elaborate for Congress and the American people about what could happen next in Syria if chemical weapons were used.
Ford declined. Perry, alluding to Iraq, said, "We don't want the current administration making the mistake of past administrations."
In fact, no consensus has emerged in Congress about what further steps should be taken to break the stalemate in Syria. Some, such as Republican Sens. John McCain and Lindsey Graham, favor strikes on Syrian air defenses, establishment of a no-fly zone and arming the opposition.
Others, like Rep. Ileana Ros-Lehtinen, R-Fla., made clear on Wednesday that providing weapons to the Syrian opposition risks having the weapons fall into the wrong hands.
"The unknown can be dangerous and the vetting of the opposition is not enough when it comes to providing lethal aid that could be used against our allies, such as Israel, or the United States in a post-Assad era," she said.