WASHINGTON—U.S. defense and diplomatic officials said Thursday that Russia is firing artillery across its border at Ukrainian military positions, an assertion that Moscow now is directly engaging in hostilities against Ukrainian government forces.

The accusation represents a harsh new U.S. stance against the Kremlin in the aftermath of the crash of Malaysia Airlines Flight 17, downed last Thursday by an antiaircraft missile. The U.S. has been accusing Russian President Vladimir Putin of supporting armed pro-Russia separatists in eastern Ukraine, but the newest allegation suggests the conflict is coming to more closely resemble a war between the two countries.(Follow the latest updates on the Malaysia Airlines crash in Ukraine.)

The latest finding, based on U.S. intelligence that officials would not detail, comes as European Union diplomats appeared Thursday to be overcoming some of the political divides that have tempered their response to the Ukrainian crisis, moving to place sanctions on Russian access to European capital markets, energy technology and sensitive military goods.

Also Thursday, the Organization for Security and Cooperation in Europe reported that "shrapnel-like, almost machine-gun-like holes" were found in two separate pieces of Flight 17's fuselage, evidence consistent with the use of a sophisticated antiaircraft missile.

Reports of Russian artillery fire into Ukraine also were picked up by social media, said a U.S. official, but were confirmed with a "high level of confidence" by U.S. intelligence agencies. U.S. officials said the information on the Russian attacks was declassified Thursday.