The United States says it has evidence that Russia is planning to deliver "heavier and more powerful" rocket launchers to pro-Russian rebels in eastern Ukraine.
"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," said Marie Harf, a spokeswoman for the State Department.
Harf said the information was based on intelligence reports.
The claim came as Russia said that America was trying to fabricate evidence that separatists in eastern Ukraine shot down Malaysia Airlines Flight MH17 on July 17.
Anatoly Antonov, the Deputy Defence Minister, said Washington's statements that pro-Russian rebels fired a missile that hit the airliner "mostly cited social networks" and did not correspond to reality.
Alexander Yakovenko, Russia's Ambassador to Britain, also poured scorn on the widespread belief that Moscow is supplying the rebels, saying: "Russia doesn't supply weapons to local de facto [separatist] authorities in eastern Ukraine" and that "no evidence whatsoever has been presented that the Russian Government has been doing this."
The ambassador made the comment after a Nato source said the alliance had seen a rising flow of arms into the conflict zone from Russia.
Antonov said US intelligence experts had claimed they could "prove the guilt of the [pro-Russian] militia and almost Russia itself" and were in possession of technical data and satellite photographs to back up their accusation.
"So where is this evidence?" he asked on the Rossiya-24 channel. "Why is it not presented to the public? Is it, if I may say so, still being finished off?"
Barack Obama, the US President, said this week that flight MH17 was brought down by a surface-to-air missile that was fired from "territory that is controlled by Russian separatists".
Samantha Powers, the US Ambassador to the United Nations, went further, adding "we cannot rule out technical assistance from Russian personnel in operating the systems".
US intelligence officials appeared to backtrack somewhat later, saying they had no definitive evidence about who fired the missile, or about Russian involvement.
However, the officials said it was implausible that Ukrainian forces fired the missile, and they still believed separatists were likely to blame.
Also yesterday, Arseniy Yatsenyuk, Ukraine's Prime Minister resigned after his governing coalition collapsed. New elections are expected to be held in October or November.
The pro-Western Yatsenyuk lashed out at the decision to pull the plug on the coalition as Kiev is struggling to end the bloody separatist insurrection in the east.
Meanwhile, in Holland, more planes arrived carrying a further 51 unidentified bodies from the MH17 crash, which killed all 298 people on board.
The Dutch Safety Board (DSB), which is in charge of investigating the crash, said that it had successfully extracted data from the aircraft's flight data recorder, or "black box", for analysis.
The recorder had not been tampered with, the DSB said.