Senior White House officials accused Russia of trying to cover up the suspected Syrian chemical attack last week, adding that the U.S. has concluded the Syrian military used banned sarin gas in the assault.

The officials also questioned whether Russia had a role in the attack and suggested it may have known that its ally Syria was planning to use sarin. In a briefing on Tuesday, they said that while there is no U.S. intelligence consensus on whether Russia had advance knowledge of the attack, it seems implausible that Moscow...