WASHINGTON — The White House accused Russia on Tuesday of engaging in a cover-up of the Syrian government’s role in a chemical weapons attack last week, saying that United States intelligence had confirmed that the Assad regime used sarin gas on its own people.

A four-page report drawn up by the National Security Council contains declassified United States intelligence on the attack and a rebuttal of Moscow’s claim that insurgents unleashed the gas to frame the Syrian government. Instead, the White House asserted that Damascus and Moscow had released “false narratives” to mislead the world.

The document also urges international condemnation of Syria’s use of chemical weapons and harshly criticizes Russia for “shielding” an ally that has used weapons of mass destruction.

The release of the dossier at a White House briefing on Tuesday marked a striking shift by President Trump, who entered office praising President Vladimir V. Putin of Russia but now appears bent on pressuring him. The accusations came as Rex W. Tillerson, the secretary of state, was preparing for meetings in Moscow on Wednesday, and as Congress and the F.B.I. are investigating potential ties between Mr. Trump’s campaign and Russia.