The United States on Thursday said it had evidence Russian forces are firing artillery from inside Russia on Ukrainian troops, in what officials called a "clear escalation" of the conflict.

Moscow is also planning to "deliver heavier and more powerful multiple rocket launchers" to the pro-Russian separatist forces in Ukraine, US deputy State Department spokeswoman Marie Harf said.

The evidence was based on "intelligence information" indicating arms were "continuing to flow across the border" into Ukraine since the shooting down of a Malaysian airliner with 298 on board, Harf said.

But she refused to reveal the evidence behind the allegation or give further information.

"They're firing artillery from within Russia to attack Ukrainian military," Harf told reporters.

Washington, however, was still looking into the downing of two Ukrainian fighter jets on Wednesday. Kiev has alleged the warplanes were hit by missiles fired from Russian territory.

The shelling by Russian forces against Ukrainian positions had been "going on for several days," said Pentagon spokesman Colonel Steven Warren.

"It's a clear escalation," Warren told reporters.

The Pentagon did not specify the precise location of the Russian artillery units or the artillery fire.

Russia has continued a troop build-up near the border of Ukraine and kept up deliveries of arms and equipment to separatists since the downing of the Malaysian airliner, US defense officials told AFP.

The Russians have sent at least one battalion a week to the border area in recent weeks, raising the troop level to 15,000 forces, up from about 12,000 last week, said two defense officials, who spoke on condition of anonymity.

"It looks like a steady increase," one official said.

Military hardware has also continued to arrive at a large base set up near Rostov, which is used as a staging and training area before the equipment is transported to the rebels in Ukraine, according to defense officials.

US intelligence officials said this week that artillery and multiple rocket launcher systems recently arrived at the southwestern base in Rostov.

At a briefing earlier this week, US intelligence officials cited commercial satellite photos that showed new structures and an apparent expansion of the base over the past month.