The United States has claimed that Russia is directing a counter offensive in support of pro-Moscow rebels in eastern Ukraine.
Separatists have seized control of territory in the east after government forces retreated from the advancing rebel troops.
Ukraine says Russian soldiers in armoured vehicles have crossed the border near where a group of paratroopers were detained earlier this week.
Kiev has also accused Russia of pushing columns of tanks and weaponry towards towns in south-eastern Ukraine.
State Department spokesman Jen Psaki said a Russian-directed counter offensive “is likely under way in Donetsk and Luhansk”.
“Clearly that is of deep concern to us,” she said.
Poland’s Prime Minister Donald Tusk also said Nato and Polish intelligence have evidence of regular Russian army units operating in Ukraine.
Russia denies any direct link to the rebels, but refuses to call on them to cease fire or disarm.
The rebels’ advance comes after weeks of government offensives saw Ukrainian troops push deep into rebel-held territory.
But the separatists’ resurgence has now prompted Kiev to call on Nato for help.
Ukraine’s Prime Minister Arseniy Yatsenyuk said it was time for Nato to act when the alliance holds a summit in Wales next week.
“We expect our Western partners and the alliance to provide practical help and take crucial decisions at the summit in September,” he said.
The fresh claims of Russian military involvement in Ukraine come after talks in Belarus ended without a major breakthrough this week.
Ukrainian President Petro Poroshenko and his Russian counterpart Vladimir Putin sat down for one-on-one talks hours after Kiev said it had captured 10 Russian paratroopers on its territory.
Mr Poroshenko said there were “some results” from the talks, but there seemed to be no significant compromises to help end four months of fighting in Ukraine.
More than 2,000 people have been killed in the conflict, with a further 400,000 people forced out of their homes.