The White House accused the Russian government on Tuesday of engaging in a cover-up of the chemical weapons attack last week by Syrian forces that prompted American missile strikes, saying that United States intelligence and numerous contemporaneous reports confirmed that the Syrians used sarin gas on their own people.

In a declassified four-page report that details United States intelligence on the chemical weapons attack and contains a point-by-point rebuttal of Moscow’s claims, the White House asserted that the Syrian and Russian governments had sought to confuse the world community about the assault through disinformation and “false narratives.”

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

It marks a striking shift by President Trump, who entered office praising President Vladimir V. Putin of Russia and seeking common ground with him — and now appears to be moving swiftly to isolate him. The charges 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 presidential campaign and Russia.

“Russia is in an island on this,” Sean Spicer, the White House press secretary, said. He said there was little dispute about Syria’s responsibility for the attack — except by Moscow and the “failed states” of North Korea and Iran.

“This is not exactly a happy-time cocktail party of people you want to be associated with,” he said. READ MORE