The top U.S. general cautioned on Sunday it remained unclear how Russia might react to the latest battlefield setbacks in Ukraine and called for increased vigilance among U.S. troops as he visited a base in Poland aiding Ukraine's war effort.
The remarks by US Army General Mark Milley, chairman of the Joint Chiefs of Staff, were a reminder of the risks of escalation as the United States and its NATO allies aid Ukraine from a distance as Kyiv wages a so-far successful counter-offensive against Russian forces.
"The war is not going too well for Russia right now. So it's incumbent upon all of us to maintain high states of readiness, alert," Milley said in Warsaw after the base visit.
Reporters travelling with him were asked not to publish the name of the base or describe its location.
Milley's trip included a review of the base's air defences, which include Patriot missile batteries that would be a last line of defense should Russia decide to attack the base -- risking war with NATO.
Milley said he was not suggesting US troops in Europe were under any increased threat but said they had to be ready.
"In the conduct of war, you just don't know with a high degree of certainty what will happen next."
The rout of Russian President Vladimir Putin's forces from northeastern Ukraine's Kharkiv region a week ago has prompted unusually strong public criticism from Russian military commentators.
Putin has brushed off the counter-offensive but said on Friday that Moscow would respond more forcefully if its troops were put under further pressure.
Milley said the war was in a new phase in which Ukrainian forces have seized the strategic initiative, although he cautioned that could change again "in weeks or months."
"Because of that, we have to very closely watch what Russia's reactions to that will be."