NATO's top commander, General Philip Breedlove, said Sunday there was a "very sizeable" Russian troop presence on the border with Ukraine and warned of a possible incursion into the Moscow-backed separatist territory of Transdniestr.

"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.

Russia's military announced Friday that it had carried out exercises in Transdniestr, a Moscow-backed separatist territory of Moldova which lies on Ukraine's southwestern border. The Russian troops are massed to the east of the country.

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.

"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."

While Moscow's military capability to intervene in Transdniestr is in no doubt, the allies "don't know about the intent", Breedlove said, adding however that Russia has been "acting much more like an adversary than a partner" since the start of the crisis in Ukraine.

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.

Russia has since maintained soldiers in the region despite a commitment in 1999 to withdraw them.