The United States claims Russia is firing artillery from its side of the border targeting Ukrainian military forces. Moscow also plans to give pro-Russian separatists heavier and more powerful weapons, says the US.
Thursday's claims from the US that Russia is firing at Ukraine military forces while planning to deliver more weapons to separatists came from US State Department spokeswoman Marie Harf, who cited intelligence reports.
"We have new evidence that the Russians intend to deliver heavier and more powerful multiple rocket launchers to separatist forces in Ukraine and have evidence that Russia is firing artillery from within Russian to attack Ukrainian military positions," Harf said.
The charges come less than two days after pro-Russian separatists shot down two Ukrainian military jets in the country's east, and a week after a Malaysian Airlines jet was brought down, which the US says was shot by a surface-to-air missile.
Last Thursday, 298 people were killed when Flight MH17 was shot down over eastern Ukraine, in an area held by Russian-backed separatists.
Russia has disputed claims it delivered heavy weapons to the separatists that would have been capable of shooting down the passenger plane. Separatists and Ukrainian government forces have blamed each other for the downing of the plane,
The bodies of 114 people who died in the disaster have already been taken to the Dutch city of Eindhoven. But heavy fighting near to the crash site meant a team of Dutch investigators leading the investigation were stuck in Kyiv on Thursday, unable to join other teams on the ground.
There are concerns that the integrity of the crash site has been severely compromised by the separatist rebels who control the area, with fears they may have interfered with evidence, particularly the wreckage.
But officials searching the plane's inflight recorders say there is no evidence either of the two black boxes were tampered with. The Dutch Safety Board (OVV) said on Wednesday that recordings from the flight data recorder had been successfully downloaded.
The recorders are being analysed in Britain after rebels handed them over to officials.
Ukraine prime minister resigns
While fighting raged close to the city of Donetsk on Wednesday, a political stalemate in Kyiv saw a frustrated Prime Minister Arseniy Yatsenyuk resign, claiming the parliament could no longer work effectively and pass laws.
The 40-year-old said his decision came after parliament's failure to pass laws that would have lessened Ukraine's dependency on Russian gas - one failed bill proposed allowing foreign investment into Ukraine's gas transportation system.
"Today's events in parliament will have dramatic consequences for the country," Yatsenyuk said.
The departure left Yatsenyuk's Fatherland party without the necessary number of seats for a majority coalition. If no new governing coalition is formed within 30 days, President Petro Poroshenko can call new elections.
jr/jm (Reuters, dpa)