The Trump administration says that they reserve right to take military action against the Syrian government if needed to prevent or deter use of chemical weapons.
They emphasized that the United States was seeking a new way to hold chemical weapons-users accountable and wanted cooperation from Russia, Assad's patron, in pressuring him to end the attacks.
"They think they can get away with it if they keep it under a certain level", an official said.
The officials weren't authorized to discuss the assessment on the record and briefed reporters on condition of anonymity.
But there have been repeated reports of smaller chemical attacks on civilian areas.
The Observatory could not confirm local reports of a chlorine attack on the Eastern Ghouta region on Thursday but it reported at least two likely uses of chlorine in attacks in January.
Assad is believed to have secretly kept part of Syria's chemical weapons stockpile despite a US-Russian deal under which Damascus was supposed to have handed over all such weapons for destruction in 2014, the US officials said.
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According to the news wire, Reuters, the prime minister and senior government figures are trapped inside the palace. Separatists stopped outside the al-Maasheeq palace, where Prime Minister Ahmed bin Daghr's government is based.
"Deterrence is something that you have to continually work at", said the second official. The Islamic State group continues to use them, they said, although the militants' arms are said to be more rudimentary.
If substantiated, the attacks would provide proof that Syria has retained a stockpile after an global deal, reached in 2013 after a chemical attack, to remove banned munitions from Syria.
Senior US officials claimed the Syrian government may be developing new types of chemical weapons.
Assad's forces have instead "evolved" their chemical weapons and made continued occasional use of them in smaller amounts since a deadly attack last April that drew a United States missile strike on a Syrian air base, the officials told reporters in a briefing.
But the officials stopped short of promising additional military action to halt Syria's use of chemical weapons.
Barrel bombs used earlier in the war to disperse chemicals indiscriminately, for example, have been replaced by ground-launched munitions, officials said.
"The recent attacks in East Ghouta raise serious concerns that Bashar al-Assad's Syrian regime might be continuing its use of chemical weapons against its own people", Tillerson, reported Voice of America.