(Adds Ukraine also accused Russia of firing rockets that downed fighter planes)

WASHINGTON, July 24 (Reuters) - The United States said on Thursday that Russia was firing artillery across the border into Ukraine to target Ukrainian military positions in the conflict against pro-Russian separatists.

"We have new evidence that the Russians intend to deliver heavier and more powerful multiple rocket launchers to the separatist forces in Ukraine, and have evidence that Russia is firing artillery from within Russia to attack Ukrainian military positions," State Department spokeswoman Marie Harf said.

Harf, speaking at a regular media briefing, cited intelligence reports, but said she could give no more information of what the reports were based on.

Russia has in the past denied it is directly involved with the rebellion in its western neighbor, but the United States and its European allies accuse Moscow of arming and encouraging the uprising and have imposed sanctions on Moscow in response.

Ukraine's Security Council said on Wednesday preliminary information indicated that missiles which brought down two government fighter jets over eastern Ukraine were fired from Russia.

Russia's Defense Ministry on Thursday dismissed this, saying it was "an attempt to mislead the public," Interfax news agency reported, citing a defense ministry official. (Reporting by David Storey; Editing by Eric Beech)

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