The US today expressed concern over the transfer of highly-sophisticated Russian weapons to Ukrainian separatists, after the Pentagon claimed that Moscow appears to be set to move more rockets across the border into Ukraine.

"We are very concerned about the transfer of weapons and materiel from the Russian side of the border into Ukraine and into the hands of Russia-backed separatists in Ukraine," White House press secretary Josh Earnest told reporters.

"That has fuelled the violence and conflict that we have seen in Ukraine. Unfortunately, it has also had tragic consequences for citizens from countries around the world after the downing of the Malaysia Airlines jetliner," he said.

Earnest said when the US announced a new set of sanctions against Russia last week, the Obama Administration noted specifically that these sanctions were aimed at Russia's continuing efforts to provide arms to the separatists.