The officials spoke on condition of anonymity because they were not authorized to brief the media.
The U.S. has so far declined to intervene in Syria's civil war, saying doing so could worsen the conflict.
On Monday, Secretary of State Hillary Rodham Clinton reiterated the Obama administration's position that Syrian use of chemical weapons was a "red line" whose crossing would prompt U.S. action.
"I'm not going to telegraph in any specifics what we would do in the event of credible evidence that the Assad regime has resorted to using chemical weapons against their own people," Clinton told reporters in Prague. "But suffice it to say, we are certainly planning to take action if that eventuality were to occur."
Syria is believed to have several hundred ballistic surface-to-surface missiles capable of carrying chemical warheads, and a U.S. defense official said American and allied intelligence officials have detected activity around more than one of Syria's chemical weapons sites in the last week.
He spoke on condition of anonymity because he was not authorized to speak to reporters about intelligence matters.
Syria's Foreign Ministry said Monday that Syria "will not use chemical weapons — if there are any — against its own people under any circumstances."
Heilprin reported from Geneva. Associated Press writers Albert Aji in Damascus, Syria, Aya Batrawy in Cairo, Bradley Klapper in Prague, Pauline Jelinek in Washington and Vladimir Isachenkov in Istanbul contributed reporting.