WASHINGTON — President Trump has drawn a new red line for President Bashar al-Assad of Syria, with American officials describing preparations at a Syrian air base for a chemical weapons assault as they sought Tuesday to bolster Mr. Trump’s threat to deter an attack.

But the administration elaborated little on the president’s unexpected, 87-word statement a night earlier that warned that Mr. Assad would “pay a heavy price” if he again released toxic gas on rebel-held territory, leaving lingering questions in Washington and in the Middle East about Mr. Trump’s intentions in Syria.

American officials have declined to rate their level of confidence about whether a chemical attack is imminent or to say whether the administration has pursued diplomatic channels to stop it. Military officials, who were initially caught off guard by Monday night’s White House statement, would not discuss what options they were considering. Conversations with allies about the chemical weapons intelligence have been kept largely secret.

In previous administrations, debates about how best to deter atrocities have played out publicly around the world. President George W. Bush took months to argue his case — later proved to be flawed — about the existence of weapons of mass destruction in Iraq. President Barack Obama offered detailed explanations about his deliberations on how to respond when Mr. Assad used chemical weapons to kill 1,400 people in 2013.