Lt Gen Ben Hodges said Russia's "disregard for civilian casualties... is not the conduct of a nation that wants to be treated like a superpower".
Russia's defence minister said on Thursday that its air force had killed 35,000 fighters in Syria.
But Russia has been accused of using heavy weapons in civilian areas.
It has consistently denied targeting civilians.
'Stronger than any potential aggressor'
Russia's aerial intervention in the Syrian conflict last year has helped the Syrian army capture eastern Aleppo. But it has further heightened tensions with the West, after it annexed the Crimean peninsula from Ukraine in March 2014.
Aleppo evacuation nears its end
Defence Minister Sergei Shoigu said 162 types of modern armaments had been tested during the campaign in Syria, which included 18,800 aerial sorties.
"What we see in Syria of course is a demonstration of capabilities and using weapons that are not necessary," Gen Hodges told the BBC.
President-elect Donald Trump is known to want better relations with Moscow, but Gen Hodges said the US military was pushing ahead with plans to bolster its presence in Europe.
"All the indications show we are going to continue our commitment," he said.
President Vladimir Putin gave a bullish account on Thursday of Russia's military strength at the defence ministry's final meeting of 2016, describing it as "currently stronger than any potential aggressor".
But he said Russia had to "strengthen the military potential of strategic nuclear forces, especially with missile complexes that can reliably penetrate any existing and prospective missile defence systems".
Mr Shoigu accused Nato of doubling the intensity of its military exercises, mainly with the focus on Russia.
He singled out the UK, accusing British armed forces of using Russian tanks and army uniforms to identify the enemy during exercises on Salisbury Plain in southern England.
"The last time this way of training troops was used was by Nazi Germany," he added.
Author: Syrian civil war