The U.S. says it has "new evidence" that Russian forces have been firing artillery across the border to attack Ukrainian military positions, and that Moscow is planning to ship powerful rocket artillery to the rebels it backs in the country's east.

"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 during a daily briefing.

"I can't tell you what the information is based on," Harf said.

The remarks come as the European Union agreed to add 15 people and 18 companies or other organizations to a sanctions list aimed at Moscow, according to Reuters.

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