The US has accused Russia of preparing to give "even more powerful rocket launchers" to Ukrainian separatists blamed for shooting down a Malaysia Airlines plane.
The rebels are said to have used a BUK missile, supplied by Moscow, to hit flight MH17 over the east of the country, mistaking it for a Ukrainian plane.
The US has warned of a significant build-up of Russian troops along the border amid fears of a possible "peacekeeping" incursion to support the increasingly embattled separatists fighting government forces.
It comes as President Barack Obama and leaders from Britain, Germany, France and Italy agreed to impose a wider set of sanctions against Russia's financial, defence and energy sectors.
Fighting is continuing in eastern UkraineUntil now, the EU sanctions have not been as tough as the American ones as Europe is more reliant on deals with Russia.
The new penalties are aimed at putting more pressure on President Vladimir Putin.
Tony Blinken, a national security adviser to Mr Obama, said: "We've seen convoys of tanks, multiple rocket launchers, artillery, and armoured vehicles.
"There's evidence it's preparing to deliver even more powerful multiple rocket launchers."
Armed pro-Russian separatists stand guard in Shakhtarsk, DonetskHe added: "We've seen a significant rebuild up of Russian forces along the border, potentially positioning Russia for a so-called humanitarian or peacekeeping intervention in Ukraine."
The US earlier released satellite images it claimed show rockets have been fired at Ukraine from within Russia.
The images, which come from the US Director of National Intelligence and have not been independently verified, also purport to show that heavy artillery for pro-Russian separatists has crossed the border.
Flight MH17 suffered a massive explosive decompression, says UkraineThe rebels deny they were responsible for the July 17 downing of the airliner, which killed all 298 people on board, including 10 Britons, whose families are set to meet Prime Minister David Cameron.
Moscow has also denied allegations of involvement in eastern Ukraine, claiming the US is conducting "an unrelenting campaign of slander against Russia, ever more relying on open lies".
Black box data from the plane reveals it crashed due to a "massive explosive decompression" after being hit by shrapnel from a missile, claims a Ukrainian security official.
And the UN has said the shooting down of the airliner may amount to a war crime.