WASHINGTON—The White House issued its strongest denunciation yet of Russian President Vladimir Putin over the downing of the Malaysia Airlines flight in eastern Ukraine, saying Friday that available evidence shows the Russians are "culpable" in the plane crash.

The charge came on the same day Pentagon officials said Moscow was preparing to transfer sophisticated rocket systems to pro-Russia separatists in Ukraine. U.S. officials said that while Russia has previously sent ground-attack rockets across the border, they have evidence that Russia was preparing to transfer heavier, more powerful rocket systems to the rebels, who have been struggling in recent weeks to counter the Ukrainian military.

A day earlier, the Obama administration charged that Russian forces have fired artillery into Ukraine in an attempt to disrupt government forces.

The U.S. criticism reflects a hardening of Washington's view that Russia supplied separatist allies with the surface-to-air weapon used to shoot down Flight 17, along with a message to European allies that Mr. Putin hasn't been embarrassed into backing down from his support of rebels.

The U.S. is trying to ratchet up pressure on Moscow, and prod European allies to take action on a broader array of sanctions to punish Russia for its support of Ukrainian rebels. Administration officials said they expect Europe to adopt more stringent sanctions against Russia within the next week, and said they are considering expanding U.S. measures. Late Friday, European officials added sanctions on individuals including Russia's director of foreign intelligence and others.