Story highlights "The violence continues to increase day by day," NATO commander warns

British security source: Fighting has returned to levels that preceded the ceasefire

Official: "Sanctions have yet to sufficiently affect Russia's calculus" on Ukraine

Gunfire crackles and flames scar battlefields in eastern Ukraine -- signs, officials warn, of a crumbling ceasefire in the volatile region.

And there's new fear that the long simmering battle may be about to explode to a deadly new level.

Gen. Philip M. Breedlove, NATO Supreme Allied Commander Europe, said he's concerned about the escalating violence and accused Moscow of violating the norms of Western civilization.

"The ceasefire is in name only at this point," he told reporters Tuesday. "The violence continues to increase day by day."

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But now, fighting between pro-Russian rebel forces and the Ukrainian military has returned to levels that preceded the ceasefire, a British security source who has detailed knowledge of the matter told CNN.

Russia has amassed some 8,000 troops along the Ukraine border, Pentagon officials and the British security source said.

Russia has also stepped up shipments of heavy weapons to separatists in recent days, Pentagon officials say. And Moscow has again sent in a convoy that it claims is humanitarian, but U.S. and Ukrainian officials say went through no independent inspection.

The situation has come up in meetings between U.S. President Barack Obama and Russian President Vladimir Putin on the sidelines of the Asia-Pacific Economic Cooperation summit meetings in Beijing.

But even the Obama administration admits that -- so far -- U.S.-led sanctions are not deterring Russia.

"The sanctions have yet to sufficiently affect Russia's calculus as it relates to Ukraine," Deputy National Security Adviser Ben Rhodes told reporters. "That's why we continue to impose them. That's why we continue to be very clear about where we need to see better Russian action."

The sanctions clearly are having an impact on Russia's economy, he said. Still, he said, "on Ukraine, we continue to be deeply troubled by Russia's actions."

The British source told CNN that so-called forced "Russification" is taking place in the Crimean region, which Russia annexed from Ukraine in March

Those who have declined Russian citizenship in Crimea are being classed as "foreigners," the sources said, and have faced difficulty in obtaining health care and banking services.

Russia argues that a majority of the region's residents voted to be ruled by Moscow. But Ukrainian leaders and many Western officials have accused Russia of violating Ukraine's sovereignty in what amounts to an illegal land grab.