The US yesterday published satellite images that it claimed were evidence of Russian artillery firing on Ukrainian forces.
The Obama administration believes the images are part of a pattern of Russian activity that demonstrates Vladimir Putin has decided to double-down on his involvement in the conflict in eastern Ukraine since the crash 10 days ago of Malaysia Airlines Flight MH17.
The US also claims that the Russian president has in recent days increased the flow of heavy weapons to separatists in eastern Ukraine. While the initial aftermath of the crash of the passenger jet raised the prospects for diplomacy, US officials say Mr Putin has instead tried to halt military gains that Ukrainian forces had been making against pro-Russian separatists.