WASHINGTON — The United States has found no evidence so far that President Bashar al-Assad’s forces have begun again using chemical weapons in Syria’s continuing civil war, the top American diplomat for Syria said on Wednesday, a day after the State Department warned of an “alleged chlorine attack” in the country’s northwest.

The diplomat, James F. Jeffrey, the special representative for Syria and a veteran Foreign Service officer, said during a congressional hearing that the Syrian military had been carrying out bombings in the area around Idlib, the last rebel stronghold, that “have been indiscriminate and very vicious.”

“We’re watching this very closely — also the reports of chemical weapons use,” he said. “So far we cannot confirm it, but we are watching it.”

Mr. Jeffrey’s testimony to the House Foreign Affairs Committee appeared to be a carefully worded recalibration of a State Department announcement the previous day on Mr. Assad’s suspected use of chemical weapons.