Washington (AFP) - The United States on Sunday released satellite images to bolster its claim that Russian artillery has fired across the border into Ukraine, targeting government forces in support of separatist rebels.

The four images, prepared by the office of the Director of National Intelligence and forwarded to reporters by the State Department, show sections of the Russia-Ukraine frontier.

Two show what the US government alleges are Russian batteries on its own territory -- one group of multiple rocket launchers and another of self-propelled howitzers.

Pictures also show impact craters near military positions on the Ukrainian side of the frontier.

A third picture shows what Washington alleges is Russian-supplied heavy weaponry being fired by separatists from within Ukraine.

"The wide area of impacts near the Ukrainian military units indicates fire from multiple rocket launchers. The bottom impact crater inset shows impacts within a local village," the caption said.

Moscow denies directly intervening in the Ukraine conflict and has accused Washington of mounting a smear campaign against it.

Washington turned up the pressure Thursday, alleging Russia had fired across the border and planned to deliver "heavier and more powerful multiple rocket launchers" to its rebel allies.

Then, the State Department cited only "intelligence information" to back up its claim before releasing the photographs on Sunday to strengthen its case.

Ukraine also claims two of its fighter jets were downed by missiles fired from Russian territory, but its US ally has yet to confirm this.

Fears of the damage that Russian arms could do in the hands of Ukraine's separatists increased sharply after a civilian Malaysian airliner was shot down over rebel territory with the loss of 298 lives.