Russia is launching artillery attacks from its soil on Ukrainian troops and preparing to move heavier weaponry across the border, the United States and Ukraine charged Friday in what appeared to be an ominous escalation of the crisis.

Russia accused Washington of lying and charged Ukraine with firing across the border on a Russian village. It also toughened its economic measures against Ukraine by banning dairy imports.

Andriy Lysenko, a spokesman for Ukraine's National Security and Defence Council, said five salvos of heavy rockets were fired across the border near the town of Kolesnikov in the Luhansk region in the country's east. A border crossing point near Marynovka was fired on twice with mortars, also from the Russian side, while Ukrainian forces shot down three Russian drones, Lysenko said.

If true, the allegations mean Moscow is playing a more direct role in the fighting than it has been accused of up to now — a dangerous turn in what is already the gravest crisis between Russia and the West since the end of the Cold War.

This is unquestionably an escalation from a military perspective. - Col. Steve Warren, Pentagon spokesman

In addition, Col. Steve Warren, a Pentagon spokesman, said that the U.S. has seen powerful rocket systems moving closer to the Ukraine border and that they could be put into the hands of the Russian-backed separatists as soon as Friday.

It wasn't clear what those developments mean for the international investigation into the downing of Malaysia Airlines Flight 17. American authorities believe the separatists shot it down with a missile, perhaps in the mistaken belief it was a military plane.

A small group of Dutch and Australian investigators combed the sprawling, unsecured field where the plane came down on July 17, taking notes and photos as their governments prepared police detachments they hope can protect the crash site and help bring the last of the 298 victims home.

U.S. officials said this week that they had new evidence that Russia intended to deliver heavier and more powerful multiple rocket launchers to the separatists. Warren said Friday the delivery could happen at any time, adding "it's that close" to the border.

Warren also corroborated Ukrainian reports of artillery fire from Russia. He said there was no indication Ukraine had shelled Russia.

"For the last several days Russian forces using Russian artillery from Russian soil have conducted attacks against Ukrainian military positions in Ukraine," Warren said. "This is unquestionably an escalation from a military perspective."

Russia's Foreign Ministry responded to U.S. allegations about cross-border shelling by saying: "Facts and details to confirm these lying contentions do not exist."

The allegations come amid a Ukrainian government offensive against the separatists that has won back control of several important towns over the past few weeks.

'Civil war by proxy'

Douglas Lute, U.S. ambassador to NATO, accused Russia of waging "civil war by proxy" in Ukraine and said the Russians have about 15,000 troops massed near the border. He spoke at a security forum in Aspen, Colorado.

In another development, CNN said pro-Russian rebels abducted one of its local freelancers on Tuesday outside the separatist-controlled city of Donetsk and was still holding him on Friday.

Anton Skiba had worked for the network for only a day when he was seized as he and other members of a CNN crew returned to their hotel from the jetliner crash site.

CNN said the abductors first accused Skiba of "terrorism" and of using his Facebook page to post cash rewards for killing separatists. Later they dropped that accusation and said he was being questioned for using identification with different last names. Later they said he had confessed to being a Ukrainian "agent."

The U.S. and human rights groups condemned Skiba's abduction as an attempt by the rebels to sow fear, CNN reported.

"We chose not to report his abduction at the time while making efforts to obtain his release," CNN spokeswoman Bridget Leininger told The Associated Press. "That has not happened to date, so we are now publicly asking those who are holding Skiba to release him immediately."

Russia said a group of its investigators came under Ukrainian mortar fire Friday in the Russian village of Primiussky. They were investigating the reported shelling two days earlier of the village, which is about 2.5 kilometres from the border. No deaths were reported.

EU considers new sanctions

In another development, European Union ambassadors reached a preliminary deal Friday on stepped-up sanctions against Russia for its involvement in Ukraine, targeting Moscow's defence and technology sectors and its access to European capital.

EU spokeswoman Maja Kocijancic said EU member states must decide whether the measures need to be approved by a summit meeting of the trade bloc's 28 member countries to go into effect.

The ambassadors also ordered asset freezes and travel bans against 15 more Russians and pro-Russian Ukrainians accused of undermining Ukraine. Eighteen businesses or other entities will also be subject to sanctions.

Russia increased its economic pressure on Ukraine when its agency in charge of agricultural products announced that it is banning imports of Ukrainian dairy. Russia is the biggest export market for Ukrainian milk and cheese.