WASHINGTON — President Trump on Sunday promised a “big price” to be paid for what he said was a chemical weapons attack that choked dozens of Syrians to death the day before, and a top White House official said the administration would not rule out a missile strike to retaliate against the government of President Bashar al-Assad.

In a tweet, Mr. Trump laid the blame for the attack partly on President Vladimir V. Putin of Russia, the first time since his election that he has criticized the Russian leader by name on Twitter. Mr. Putin’s forces have been fighting for years to keep the Assad government in power amid Syria’s brutal civil war.

Mr. Trump also left no doubt that he believed the assessment of aid groups that Mr. Assad’s military had used chemical weapons to inflict the carnage on Saturday in Douma, a rebel-held suburb of Damascus. The attack left at least 42 people dead in their homes from apparent suffocation and sent many others to clinics with burning eyes and breathing problems.

“Many dead, including women and children, in mindless CHEMICAL attack in Syria,” Mr. Trump wrote. “President Putin, Russia and Iran are responsible for backing Animal Assad.”