An already fragile truce in eastern Ukraine is under increasing strain, as combatants have begun using the sort of heavy weaponry that a peace deal negotiated in February had banned from the battlefield. And a major part of the problem, according to the U.S. ambassador to Ukraine, Geoffrey Pyatt, is Russia’s increasing involvement in the conflict.

On Tuesday morning Pyatt, via Twitter, said that the U.S. believes that Russia is violating both the spirit and the letter of the peace agreement by training rebel forces in the use of heavy weaponry and maintaining advanced heavy weapons systems on the front lines after agreeing to withdraw them.

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1/4 Russia has command and control elements in eastern Ukraine to coordinate military operations there. — Geoffrey Pyatt (@GeoffPyatt) April 14, 2015

2/4 Russia continues to provide support to the separatists. — Geoffrey Pyatt (@GeoffPyatt) April 14, 2015

3/4 We know Russia established training areas in eastern Ukraine to train separatist fighters in gunnery and artillery firing. — Geoffrey Pyatt (@GeoffPyatt) April 14, 2015

4/4 Russia continues to maintain advanced surface-to-air missile systems near the front lines in violation of the Minsk agreements. — Geoffrey Pyatt (@GeoffPyatt) April 14, 2015

Russian officials, including President Vladimir Putin, have continually asserted that there is no significant Russian presence in Ukraine, and that any Russian troops there are volunteers who are on vacation. Other governments, including the countries of NATO, as well as multiple non-government organizations have all said that the proof of Russia’s continued involvement in the Ukrainian conflict is overwhelming and incontrovertible.

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Overnight and into Tuesday morning, there was heavy shelling reported in the key city of Donetsk, just a day after negotiators on both sides had agreed to further deescalate the situation by removing more weapons from the front lines.

A spokesperson for the government in Kiev said that in the space of 24 hours, six of its soldiers were killed and another dozen wounded.

The new flare-up follows several weeks of relative calm in the region, where more than 6,000 people have been killed in the past year. Rebels began claiming territory in eastern Ukraine almost a year ago, after Russia invaded Ukraine’s Crimean peninsula and ultimately claimed it as part of Russia.

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On Tuesday, German Foreign Minister Frank-Walter Steinmeier signaled that the European community remains determined to punish Russia economically for its invasion of Ukrainian territory and its continued support of the armed rebellion in the east.

Steinmeier said that Russia would continue to be excluded from high-level economic negotiations between major world powers until a settlement to the Ukrainian conflict is reached.

“We have the situation we have because Russia decided to violate international law,” Steinmeier said.

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