"The force that is at the Ukrainian border now to the east is very, very sizeable and very, very ready," the head of the US European Command told the Brussels Forum.
"What I think is worrisome is that Russia has used these series of snap exercises to sort of condition us," he added at the forum organised by the US think tank the German Marshall Fund.
NATO has previously observed several large-scale exercises by the Russian army, in which the "forces were brought to readiness and exercised and then they stood down," Breedlove said.
"Then, as we have all seen, a snap exercise, large formation brought to readiness and, boom, into Crimea we (Russians) went with a highly ready, highly prepared force," he said.
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"There is absolutely sufficient force postured on the eastern border of Ukraine to run to Transdniestr if the decision was made (in Moscow) to do that," Breedlove said, adding: "And that's very worrisome."
The games in Transdniestr came after pro-Western Moldovan President Nicolae Timofti on Tuesday voiced concerns that the Crimean scenario could play out in his mainly Russian-speaking country.
He said the spokesman of Transdniestr's self-appointed assembly had asked Russia to absorb the region along with Crimea.
The eastern Moldovan region, which is dominated by Russian and Ukrainian minorities, seceded with Moscow's support in 1992 but has never been recognised by any other country.