The United States has "evidence" that Russia is firing artillery from within its territory to attack Ukrainian military troops, a State Department representative said Thursday.
Spokeswoman Marie Harf said the charges were based on new intelligence information. She said the U.S. also has evidence Russia intends to supply "heavier, more powerful rocket launchers to separatists,” but declined to provide further specifics.
Last Thursday, a Malaysia Airlines flight was shot down over eastern Ukraine, allegedly by pro-Russian militants, killing all 298 people on board.
U.S. officials say the flight was brought down by a surface-to-air missile fired from territory controlled by the militants and have accused Russia of providing those groups with weaponry and other assistance.
The charges are the latest as the Obama administration looks to build international support for tougher sanctions on Russia. Officials also say the U.S. is weighing additional measures on its own.
"We're assessing all the sorts of tool in our arsenal as we move forward," White House spokesman Eric Schultz said on Wednesday.
Harf added that the U.S. is still looking into whether artillery fired from Russian territory had taken down two Ukrainian military planes on Wednesday.
Schultz on Thursday said the administration also welcomed reports that the European Union is considering broader sanctions.
Russia's ambassador to London said Thursday that Russian had never given weapons to separatists, The Guardian reported.
"The case, as is admitted, is built upon photos and messages from social media sites, placed by Ukrainian authorities and since then proved to be forgeries," Alexander Yakovenko told journalists at the Russian embassy in London.
"Naturally, our American partners say that they have no way of certifying the authenticity of those materials."
Yakovenko said Russian officials were providing humanitarian assistance and taking in Ukrainian refugees.
Pro-Russian separatists and Ukraine security forces have clashed since March, when Russia annexed the Ukrainian peninsula of Crimea.
Yakovenko said the war in Ukraine had created "murky waters which are a fertile ground for all sorts of incidents."
Kristina Wong contributed.
This story was updated at 4:54 p.m.