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 Ukraine Until 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, Donetsk He 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 Ukraine The 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.
Amiran Gugutishvili whose orchard has been fenced off
David Vanishvili, 80, who finds himself in South Ossetia
Georgia and South Ossetia (borders are approximate and some are disputed)
A Palestinian girl at the scene of an explosion that killed nine children
The Al Aqsa TV headquaters after being hit by an Israeli shell
An Israeli soldier hurt in a mortar attack is stretchered away by comrades
A building within Gaza port is seen on fire after several missile strikes
Israeli soldiers with the coffin of their comrade Liad Lavi
Lavi died from wounds sustained last week while fighting in Gaza
Firefighters in Gaza battle a fire started after a rocket attack
The couple appeared emotional at the crash scene
A section of the plane's fuselage
Israel has resumed its offensive in Gaza after a fragile truce
Some 132 bodies were pulled from rubble in Gaza during Saturday's truce
Israelis demonstrated against their government's military operation
President Putin is coming under increasing international pressure
This map shows the flight path of MH17 before it crashed