The Kremlin said Friday it had deployed armed forces near Ukraine to shore up its porous border, after NATO voiced concern about a Russian troop buildup in the area.
Ukraine has accused Russia of sending fighters and arms across the long border to bolster a separatist rebellion while Moscow has also accused Kiev of violating the frontier.
President Vladimir Putin's spokesman said the troop movements were aimed at buttressing the border and had been planned weeks ago.
"In this case we cannot speak of any concentration of troops other than measures to reinforce the protection of Russia's borders, which are being carried out on President Vladimir Putin's direct orders," spokesman Dmitry Peskov said.
The order "was given several weeks ago" because there have been "more incidents" of border breaches, he was quoted as saying by Russian news agencies.
NATO chief Anders Fogh Rasmussen said on Thursday that Russia had sent a few thousand more troops to the border, calling it a "very regrettable step backwards".
Putin's top foreign policy aide Yury Ushakov denied any troop increases, saying it was "Russia's right" to reinforce the border following a string of violations.
The federal security service (FSB) which runs the border service in Russia, said all border control checkpoints were working normally except one, which was closed unilaterally by the Ukrainian side.
Fighting between pro-Russian rebels and government troops in east Ukraine has killed hundreds since April and sent thousands fleeing across the border.
Russia on Friday said it began a drill moving some troops from western Siberia and the Urals to other places, without specifying where.
A defence ministry source told RBK news agency this week that troops were prepared to enter Ukraine's insurgent regions in order to "put up barriers between the civilian population and the Ukrainian army".